dt-bindings: sound: Document jz4740-codec bindings

Ingenic JZ4740 codec controller

Required properties:

- compatible : "ingenic,jz4740-codec"

- reg : codec registers location and length

- clocks : phandle to the AIC clock.

- clock-names: must be set to "aic".

- #sound-dai-cells: Must be set to 0.

Example:

codec: audio-codec@10020080 {

	compatible = "ingenic,jz4740-codec";

	reg = <0x10020080 0x8>;

	#sound-dai-cells = <0>;

	clocks = <&cgu JZ4740_CLK_AIC>;

	clock-names = "aic";

};