dt: Add documentation for the ZTE SPDIF controller

This patch adds the devicetree documentation for the ZTE
zx296702 SPDIF audio controller.
Signed-off-by: Jun Nie <jun.nie@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/zte,zx-spdif.txt

+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";
+	};