dt: Add bindings documentation for the ADI AXI-SPDIF audio controller

ADI AXI-SPDIF controller

Required properties:

 - compatible : Must be "adi,axi-spdif-1.00.a"

 - reg : Must contain SPDIF core's registers location and length

 - clocks : Pairs of phandle and specifier referencing the controller's clocks.

   The controller expects two clocks, the clock used for the AXI interface and

   the clock used as the sampling rate reference clock sample.

 - clock-names: "axi" for the clock to the AXI interface, "ref" for the sample

   rate reference clock.

 - dmas: Pairs of phandle and specifier for the DMA channel that is used by

   the core. The core expects one dma channel for transmit.

 - dma-names : Must be "tx"

For more details on the 'dma', 'dma-names', 'clock' and 'clock-names' properties

please check:

	* resource-names.txt

	* clock/clock-bindings.txt

	* dma/dma.txt

Example:

	spdif: spdif@0x77400000 {

		compatible = "adi,axi-spdif-tx-1.00.a";

		reg = <0x77600000 0x1000>;

		clocks = <&clk 15>, <&audio_clock>;

		clock-names = "axi", "ref";

		dmas = <&ps7_dma 0>;

		dma-names = "tx";

	};