ASoC: tas2552: Improve DT binding document example

Improve DT binding document example by providing the mandatory
regulator properties.
Signed-off-by: NFabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/tas2552.txt

@@ -25,6 +25,9 @@ Example:
 tas2552: tas2552@41 {
 	compatible = "ti,tas2552";
 	reg = <0x41>;
+	vbat-supply = <&reg_vbat>;
+	iovdd-supply = <&reg_iovdd>;
+	avdd-supply = <&reg_avdd>;
 	enable-gpio = <&gpio4 2 GPIO_ACTIVE_HIGH>;
 };