dt: Add documentation for the ZTE SPDIF controller

ZTE ZX296702 SPDIF controller

Required properties:

 - compatible : Must be "zte,zx296702-spdif"

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

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

 - clock-names: "tx" for the clock to the SPDIF interface.

 - 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:

	spdif0: spdif0@0b004000 {

		compatible = "zte,zx296702-spdif";

		reg = <0x0b004000 0x1000>;

		clocks = <&lsp0clk ZX296702_SPDIF0_DIV>;

		clock-names = "tx";

		interrupts = <GIC_SPI 21 IRQ_TYPE_LEVEL_HIGH>;

		dmas = <&dma 4>;

		dma-names = "tx";

		status = "okay";

	};