ASoC: meson: add axg pdm input DT binding documentation

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

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

+* Amlogic Audio PDM input
+
+Required properties:
+- compatible: 'amlogic,axg-pdm'
+- reg: physical base address of the controller and length of memory
+       mapped region.
+- clocks: list of clock phandle, one for each entry clock-names.
+- clock-names: should contain the following:
+  * "pclk"   : peripheral clock.
+  * "dclk"   : pdm digital clock
+  * "sysclk" : dsp system clock
+- #sound-dai-cells: must be 0.
+
+Example of PDM on the A113 SoC:
+
+pdm: audio-controller@ff632000 {
+	compatible = "amlogic,axg-pdm";
+	reg = <0x0 0xff632000 0x0 0x34>;
+	#sound-dai-cells = <0>;
+	clocks = <&clkc_audio AUD_CLKID_PDM>,
+		 <&clkc_audio AUD_CLKID_PDM_DCLK>,
+		 <&clkc_audio AUD_CLKID_PDM_SYSCLK>;
+	clock-names = "pclk", "dclk", "sysclk";
+};