ASoC: meson: add axg tdm formatters DT bindings documentation

* Amlogic Audio TDM formatters

Required properties:

- compatible: 'amlogic,axg-tdmin' or

	      'amlogic,axg-tdmout'

- 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.

  * "sclk"     : bit clock.

  * "sclk_sel" : bit clock input multiplexer.

  * "lrclk"    : sample clock

  * "lrclk_sel": sample clock input multiplexer

Example of TDMOUT_A on the A113 SoC:

tdmout_a: audio-controller@500 {

	compatible = "amlogic,axg-tdmout";

	reg = <0x0 0x500 0x0 0x40>;

	clocks = <&clkc_audio AUD_CLKID_TDMOUT_A>,

		 <&clkc_audio AUD_CLKID_TDMOUT_A_SCLK>,

		 <&clkc_audio AUD_CLKID_TDMOUT_A_SCLK_SEL>,

		 <&clkc_audio AUD_CLKID_TDMOUT_A_LRCLK>,

		 <&clkc_audio AUD_CLKID_TDMOUT_A_LRCLK>;

	clock-names = "pclk", "sclk", "sclk_sel",

		      "lrclk", "lrclk_sel";

};