dt-bindings: ASoC: xlnx, spdif: Document spdif bindings

Added documentation for SPDIF IP DT bindings.
Signed-off-by: NMaruthi Srinivas Bayyavarapu <maruthi.srinivas.bayyavarapu@xilinx.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/xlnx,spdif.txt

+Device-Tree bindings for Xilinx SPDIF IP
+
+The IP supports playback and capture of SPDIF audio
+
+Required properties:
+ - compatible: "xlnx,spdif-2.0"
+ - clock-names: List of input clocks.
+   Required elements: "s_axi_aclk", "aud_clk_i"
+ - clocks: Input clock specifier. Refer to common clock bindings.
+ - reg: Base address and address length of the IP core instance.
+ - interrupts-parent: Phandle for interrupt controller.
+ - interrupts: List of Interrupt numbers.
+ - xlnx,spdif-mode: 0 :- receiver mode
+		    1 :- transmitter mode
+ - xlnx,aud_clk_i: input audio clock value.
+
+Example:
+	spdif_0: spdif@80010000 {
+		clock-names = "aud_clk_i", "s_axi_aclk";
+		clocks = <&misc_clk_0>, <&clk 71>;
+		compatible = "xlnx,spdif-2.0";
+		interrupt-names = "spdif_interrupt";
+		interrupt-parent = <&gic>;
+		interrupts = <0 91 4>;
+		reg = <0x0 0x80010000 0x0 0x10000>;
+		xlnx,spdif-mode = <1>;
+		xlnx,aud_clk_i = <49152913>;
+	};