ASoC: meson: add axg pdm input DT binding documentation

* 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";

};