ASoC: dt-bindings: qcom,wcd939x: Document missing VDD_PX supply

Document VDD_PX supply on WCD9390 and WCD9395 audio codecs, which was
missed in original posting.

Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Acked-by: "Rob Herring (Arm)" <[e-mail]>
Reviewed-by: Srinivas Kandagatla <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/sound/qcom,wcd939x.yaml

···
       purpose of handling altmode muxing and orientation switching to detect and
       enable Audio Accessory Mode.
 
+  vdd-px-supply:
+    description: A reference to the 1.2V PX supply
+
 required:
   - compatible