ASoC: add TA5720 digital amplifier DT bindings

The Texas Instruments TAS5720L/M device is a high-efficiency mono
Class-D audio power amplifier optimized for high transient power
capability to use the dynamic power headroom of small loudspeakers.
Its digital time division multiplexed (TDM) interface enables up to
16 devices to share the same bus.
Signed-off-by: NAndreas Dannenberg <dannenberg@ti.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/tas5720.txt

+Texas Instruments TAS5720 Mono Audio amplifier
+
+The TAS5720 serial control bus communicates through the I2C protocol only. The
+serial bus is also used for periodic codec fault checking/reporting during
+audio playback. For more product information please see the links below:
+
+http://www.ti.com/product/TAS5720L
+http://www.ti.com/product/TAS5720M
+
+Required properties:
+
+- compatible : "ti,tas5720"
+- reg : I2C slave address
+- dvdd-supply : phandle to a 3.3-V supply for the digital circuitry
+- pvdd-supply : phandle to a supply used for the Class-D amp and the analog
+
+Example:
+
+tas5720: tas5720@6c {
+	status = "okay";
+	compatible = "ti,tas5720";
+	reg = <0x6c>;
+	dvdd-supply = <&vdd_3v3_reg>;
+	pvdd-supply = <&amp_supply_reg>;
+};