ASoC: dt-bindings: ak5558: Add power supply property

AVDD-supply is for Analog power supply
DVDD-supply is for Digital power supply
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/1598190877-9213-1-git-send-email-shengjiu.wang@nxp.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/ak5558.txt

@@ -10,6 +10,8 @@ Required properties:
 Optional properties:
 
 - reset-gpios: A GPIO specifier for the power down & reset pin.
+- AVDD-supply: Analog power supply
+- DVDD-supply: Digital power supply
 
 Example: