spi/mxs: Add SSP/SPI device tree documentation

* Freescale MX233/MX28 SSP/SPI

Required properties:

- compatible: Should be "fsl,<soc>-spi", where soc is "imx23" or "imx28"

- reg: Offset and length of the register set for the device

- interrupts: Should contain SSP interrupts (error irq first, dma irq second)

- fsl,ssp-dma-channel: APBX DMA channel for the SSP

Example:

ssp0: ssp@80010000 {

	#address-cells = <1>;

	#size-cells = <0>;

	compatible = "fsl,imx28-spi";

	reg = <0x80010000 0x2000>;

	interrupts = <96 82>;

	fsl,ssp-dma-channel = <0>;

};