spi: Document DT bindings for SPI controllers in slave mode

SPI (Serial Peripheral Interface) busses

SPI busses can be described with a node for the SPI master device

and a set of child nodes for each SPI slave on the bus.  For this

discussion, it is assumed that the system's SPI controller is in

SPI master mode.  This binding does not describe SPI controllers

in slave mode.

SPI busses can be described with a node for the SPI controller device

and a set of child nodes for each SPI slave on the bus.  The system's SPI

controller may be described for use in SPI master mode or in SPI slave mode,

but not for both at the same time.

The SPI master node requires the following properties:

The SPI controller node requires the following properties:

- compatible      - Name of SPI bus controller following generic names

		    recommended practice.

In master mode, the SPI controller node requires the following additional

properties:

- #address-cells  - number of cells required to define a chip select

		address on the SPI bus.

- #size-cells     - should be zero.

- compatible      - name of SPI bus controller following generic names

		recommended practice.

In slave mode, the SPI controller node requires one additional property:

- spi-slave       - Empty property.

No other properties are required in the SPI bus node.  It is assumed

that a driver for an SPI bus device will understand that it is an SPI bus.

However, the binding does not attempt to define the specific method for

chip selects.  Individual drivers can define additional properties to

support describing the chip select layout.

Optional properties:

Optional properties (master mode only):

- cs-gpios	  - gpios chip select.

- num-cs	  - total number of chipselects.

cs2 : &gpio1 1 0

cs3 : &gpio1 2 0

SPI slave nodes must be children of the SPI master node and can

contain the following properties.

- reg             - (required) chip select address of device.

- compatible      - (required) name of SPI device following generic names

		recommended practice.

- spi-max-frequency - (required) Maximum SPI clocking speed of device in Hz.

- spi-cpol        - (optional) Empty property indicating device requires

		inverse clock polarity (CPOL) mode.

- spi-cpha        - (optional) Empty property indicating device requires

		shifted clock phase (CPHA) mode.

- spi-cs-high     - (optional) Empty property indicating device requires

		chip select active high.

- spi-3wire       - (optional) Empty property indicating device requires

		3-wire mode.

- spi-lsb-first   - (optional) Empty property indicating device requires

		LSB first mode.

- spi-tx-bus-width - (optional) The bus width (number of data wires) that is

                      used for MOSI. Defaults to 1 if not present.

- spi-rx-bus-width - (optional) The bus width (number of data wires) that is

                      used for MISO. Defaults to 1 if not present.

- spi-rx-delay-us  - (optional) Microsecond delay after a read transfer.

- spi-tx-delay-us  - (optional) Microsecond delay after a write transfer.

SPI slave nodes must be children of the SPI controller node.

In master mode, one or more slave nodes (up to the number of chip selects) can

be present.  Required properties are:

- compatible      - Name of SPI device following generic names recommended

		    practice.

- reg             - Chip select address of device.

- spi-max-frequency - Maximum SPI clocking speed of device in Hz.

In slave mode, the (single) slave node is optional.

If present, it must be called "slave".  Required properties are:

- compatible      - Name of SPI device following generic names recommended

		    practice.

All slave nodes can contain the following optional properties:

- spi-cpol        - Empty property indicating device requires inverse clock

		    polarity (CPOL) mode.

- spi-cpha        - Empty property indicating device requires shifted clock

		    phase (CPHA) mode.

- spi-cs-high     - Empty property indicating device requires chip select

		    active high.

- spi-3wire       - Empty property indicating device requires 3-wire mode.

- spi-lsb-first   - Empty property indicating device requires LSB first mode.

- spi-tx-bus-width - The bus width (number of data wires) that is used for MOSI.

		    Defaults to 1 if not present.

- spi-rx-bus-width - The bus width (number of data wires) that is used for MISO.

		    Defaults to 1 if not present.

- spi-rx-delay-us - Microsecond delay after a read transfer.

- spi-tx-delay-us - Microsecond delay after a write transfer.

Some SPI controllers and devices support Dual and Quad SPI transfer mode.

It allows data in the SPI system to be transferred using 2 wires (DUAL) or 4