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
- spi-slave: Empty property indicating the SPI controller is used in slave mode.
- ready-gpios: GPIO used to signal a SPI master that the FIFO is filled
  and we're ready to service a transfer. Only useful in slave mode.

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>;
	};