ASoC: meson: add axg fifos DT binding documentation

* Amlogic Audio FIFO controllers

Required properties:

- compatible: 'amlogic,axg-toddr' or

	      'amlogic,axg-frddr'

- reg: physical base address of the controller and length of memory

       mapped region.

- interrupts: interrupt specifier for the fifo.

- clocks: phandle to the fifo peripheral clock provided by the audio

	  clock controller.

- resets: phandle to memory ARB line provided by the arb reset controller.

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

Example of FRDDR A on the A113 SoC:

frddr_a: audio-controller@1c0 {

	compatible = "amlogic,axg-frddr";

	reg = <0x0 0x1c0 0x0 0x1c>;

	#sound-dai-cells = <0>;

	interrupts = <GIC_SPI 88 IRQ_TYPE_EDGE_RISING>;

	clocks = <&clkc_audio AUD_CLKID_FRDDR_A>;

	resets = <&arb AXG_ARB_FRDDR_A>;

};