ASoC: mediatek: mt6358: add codec document

Mediatek MT6358 Audio Codec

The communication between MT6358 and SoC is through Mediatek PMIC wrapper.

For more detail, please visit Mediatek PMIC wrapper documentation.

Must be a child node of PMIC wrapper.

Required properties:

- compatible : "mediatek,mt6358-sound".

- Avdd-supply : power source of AVDD

Example:

mt6358_snd {

	compatible = "mediatek,mt6358-sound";

	Avdd-supply = <&mt6358_vaud28_reg>;

};