spi: Document the STM32 SPI bindings

STMicroelectronics STM32 SPI Controller

The STM32 SPI controller is used to communicate with external devices using

the Serial Peripheral Interface. It supports full-duplex, half-duplex and

simplex synchronous serial communication with external devices. It supports

from 4 to 32-bit data size. Although it can be configured as master or slave,

only master is supported by the driver.

Required properties:

- compatible: Must be "st,stm32-spi".

- reg: Offset and length of the device's register set.

- interrupts: Must contain the interrupt id.

- clocks: Must contain an entry for spiclk (which feeds the internal clock

	  generator).

- #address-cells:  Number of cells required to define a chip select address.

- #size-cells: Should be zero.

Optional properties:

- resets: Must contain the phandle to the reset controller.

- A pinctrl state named "default" may be defined to set pins in mode of

  operation for SPI transfer.

- dmas: DMA specifiers for tx and rx dma. DMA fifo mode must be used. See the

  STM32 DMA bindings, Documentation/devicetree/bindings/dma/stm32-dma.txt.

- dma-names: DMA request names should include "tx" and "rx" if present.

- cs-gpios: list of GPIO chip selects. See the SPI bus bindings,

  Documentation/devicetree/bindings/spi/spi-bus.txt

Child nodes represent devices on the SPI bus

  See ../spi/spi-bus.txt

Optional properties:

- st,spi-midi-ns: (Master Inter-Data Idleness) minimum time delay in

		  nanoseconds inserted between two consecutive data frames.

Example:

	spi2: spi@40003800 {

		#address-cells = <1>;

		#size-cells = <0>;

		compatible = "st,stm32-spi";

		reg = <0x40003800 0x400>;

		interrupts = <36>;

		clocks = <&rcc SPI2_CK>;

		resets = <&rcc 1166>;

		dmas = <&dmamux1 0 39 0x400 0x01>,

		       <&dmamux1 1 40 0x400 0x01>;

		dma-names = "rx", "tx";

		pinctrl-0 = <&spi2_pins_b>;

		pinctrl-names = "default";

		status = "okay";

		cs-gpios = <&gpioa 11 0>;

		spidev@0 {

			compatible = "spidev";

			reg = <0>;

			spi-max-frequency = <4000000>;

			st,spi-midi = <4000>;

		};

	};