spi/mxs: Add SSP/SPI device tree documentation

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Chris Ball <cjb@laptop.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

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

@@ -0,0 +1,18 @@
+* 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>;
+};