ASoC: img: Add binding document for Pistachio internal DAC

Add binding document for Pistachio Internal DAC
Signed-off-by: NDamien.Horsley <Damien.Horsley@imgtec.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/img,pistachio-internal-dac.txt

+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>;
+};