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

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>;

	};