spi: Add devicetree bindings documentation for Cadence SPI

Add spi-cadence bindings documentation.
Signed-off-by: Harini Katakam <harinik@xilinx.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

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

+Cadence SPI controller Device Tree Bindings
+-------------------------------------------
+
+Required properties:
+- compatible		: Should be "cdns,spi-r1p6" or "xlnx,zynq-spi-r1p6".
+- reg			: Physical base address and size of SPI registers map.
+- interrupts		: Property with a value describing the interrupt
+			  number.
+- interrupt-parent	: Must be core interrupt controller
+- clock-names		: List of input clock names - "ref_clk", "pclk"
+			  (See clock bindings for details).
+- clocks		: Clock phandles (see clock bindings for details).
+
+Optional properties:
+- num-cs		: Number of chip selects used.
+			  If a decoder is used, this will be the number of
+			  chip selects after the decoder.
+- is-decoded-cs		: Flag to indicate whether decoder is used or not.
+
+Example:
+
+	spi@e0007000 {
+		compatible = "xlnx,zynq-spi-r1p6";
+		clock-names = "ref_clk", "pclk";
+		clocks = <&clkc 26>, <&clkc 35>;
+		interrupt-parent = <&intc>;
+		interrupts = <0 49 4>;
+		num-cs = <4>;
+		is-decoded-cs = <0>;
+		reg = <0xe0007000 0x1000>;
+	} ;