ASoC: rt5663: Add documentation for power supply support

rt5663 codec driver will support setting CPVDD and AVDD power supply
from device tree.
Signed-off-by: Cheng-Yi Chiang <cychiang@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/rt5663.txt

@@ -10,6 +10,10 @@ Required properties:
 
 - interrupts : The CODEC's interrupt output.
 
+- avdd-supply: Power supply for AVDD, providing 1.8V.
+
+- cpvdd-supply: Power supply for CPVDD, providing 3.5V.
+
 Optional properties:
 
 - "realtek,dc_offset_l_manual"
@@ -51,4 +55,6 @@ rt5663: codec@12 {
 	compatible = "realtek,rt5663";
 	reg = <0x12>;
 	interrupts = <7 IRQ_TYPE_EDGE_FALLING>;
+	avdd-supply = <&pp1800_a_alc5662>;
+	cpvdd-supply = <&pp3500_a_alc5662>;
 };