ASoC: wm8804: Update DT binding document to cover regulator supplies

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/wm8804.txt

@@ -10,6 +10,9 @@ Required properties:
   - reg : the I2C address of the device for I2C, the chip select
           number for SPI.
 
+  - PVDD-supply, DVDD-supply : Power supplies for the device, as covered
+    in Documentation/devicetree/bindings/regulator/regulator.txt
+
 Example:
 
 codec: wm8804@1a {