Atmel SPI device

Required properties:
- compatible : should be "atmel,at91rm9200-spi".
- reg: Address and length of the register set for the device
- interrupts: Should contain spi interrupt
- cs-gpios: chipselects (optional for SPI controller version >= 2 with the
  Chip Select Active After Transfer feature).
- clock-names: tuple listing input clock names.
	Required elements: "spi_clk"
- clocks: phandles to input clocks.

Optional properties:
- atmel,fifo-size: maximum number of data the RX and TX FIFOs can store for FIFO
  capable SPI controllers.

Example:

spi1: spi@fffcc000 {
	compatible = "atmel,at91rm9200-spi";
	reg = <0xfffcc000 0x4000>;
	interrupts = <13 4 5>;
	#address-cells = <1>;
	#size-cells = <0>;
	clocks = <&spi1_clk>;
	clock-names = "spi_clk";
	cs-gpios = <&pioB 3 0>;
	atmel,fifo-size = <32>;
	status = "okay";

	mmc-slot@0 {
		compatible = "mmc-spi-slot";
		reg = <0>;
		gpios = <&pioC 4 0>;	/* CD */
		spi-max-frequency = <25000000>;
	};
};