ASoC: meson: add axg spdif output DT bindings documentation

* Amlogic Audio SPDIF Output

Required properties:

- compatible: 'amlogic,axg-spdifout'

- clocks: list of clock phandle, one for each entry clock-names.

- clock-names: should contain the following:

  * "pclk" : peripheral clock.

  * "mclk" : master clock

- #sound-dai-cells: must be 0.

Example on the A113 SoC:

spdifout: audio-controller@480 {

	compatible = "amlogic,axg-spdifout";

	reg = <0x0 0x480 0x0 0x50>;

	#sound-dai-cells = <0>;

	clocks = <&clkc_audio AUD_CLKID_SPDIFOUT>,

		 <&clkc_audio AUD_CLKID_SPDIFOUT_CLK>;

	clock-names = "pclk", "mclk";

};