ASoC: dt-bindings: rt5645: add suppliers

The rt5645 and rt5650 have two suppliers, document them.
Signed-off-by: NMatthias Brugger <matthias.bgg@gmail.com>

Link: https://lore.kernel.org/r/20200114150151.8537-1-matthias.bgg@kernel.orgSigned-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/rt5645.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:
 
 - hp-detect-gpios: