ASoC: img: Add binding document for Pistachio internal DAC

Pistachio internal DAC DT bindings

Required properties:

  - compatible: "img,pistachio-internal-dac"

  - img,cr-top : Must contain a phandle to the top level control syscon

		 node which contains the internal dac control registers

  - VDD-supply : Digital power supply regulator (+1.8V or +3.3V)

Examples:

internal_dac: internal-dac {

	compatible = "img,pistachio-internal-dac";

	img,cr-top = <&cr_top>;

	VDD-supply = <&supply3v3>;

};