spi/spi-pxa2xx: add PXA2xx SSP SPI Controller

PXA2xx SSP SPI Controller

Required properties:

- compatible: Must be "marvell,mmp2-ssp".

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

- interrupts: Should be the interrupt number.

- clocks: Should contain a single entry describing the clock input.

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

- #size-cells: Should be zero.

Optional properties:

- 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

Example:

	ssp1: spi@d4035000 {

		compatible = "marvell,mmp2-ssp";

		reg = <0xd4035000 0x1000>;

		clocks = <&soc_clocks MMP2_CLK_SSP0>;

		interrupts = <0>;

	};