ASoC: meson: add axg spdif input DT binding documentation

* 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";

};