ASoC: meson: add axg spdif input DT binding documentation

Add the DT binding documentation for axg's SPDIF input.
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/amlogic,axg-spdifin.txt

+* Amlogic Audio SPDIF Input
+
+Required properties:
+- compatible: 'amlogic,axg-spdifin'
+- interrupts: interrupt specifier for the spdif input.
+- clocks: list of clock phandle, one for each entry clock-names.
+- clock-names: should contain the following:
+  * "pclk" : peripheral clock.
+  * "refclk" : spdif input reference clock
+- #sound-dai-cells: must be 0.
+
+Example on the A113 SoC:
+
+spdifin: audio-controller@400 {
+	compatible = "amlogic,axg-spdifin";
+	reg = <0x0 0x400 0x0 0x30>;
+	#sound-dai-cells = <0>;
+	interrupts = <GIC_SPI 87 IRQ_TYPE_EDGE_RISING>;
+	clocks = <&clkc_audio AUD_CLKID_SPDIFIN>,
+		 <&clkc_audio AUD_CLKID_SPDIFIN_CLK>;
+	clock-names = "pclk", "refclk";
+};