ASoC: dt-bindings: add regulator support to dmic codec

Add optional vref-supply property to describe the regulator supplying
the digital microphones.

Signed-off-by: Olivier Moysan <[e-mail]>
Acked-by: "Rob Herring (Arm)" <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/sound/dmic-codec.yaml

···
   '#sound-dai-cells':
     const: 0
 
+  vref-supply:
+    description: Phandle to the digital microphone reference supply
+
   dmicen-gpios:
     description: GPIO specifier for DMIC to control start and stop
     maxItems: 1