ASoC: dt-bindings: sgtl5000: Add missing type to 'micbias-voltage-m-volts'

'micbias-voltage-m-volts' is missing a type definition. '-m-volts' is
not a standard unit (should be '-microvolt'). As the property is already
in use, add a type reference.
Signed-off-by: NRob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220719215134.1877363-1-robh@kernel.orgSigned-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/sgtl5000.yaml

@@ -47,6 +47,7 @@ properties:
     description: The bias voltage to be used in mVolts. The voltage can take
       values from 1.25V to 3V by 250mV steps. If this node is not mentioned
       or the value is unknown, then the value is set to 1.25V.
+    $ref: "/schemas/types.yaml#/definitions/uint32"
     enum: [ 1250, 1500, 1750, 2000, 2250, 2500, 2750, 3000 ]
 
   lrclk-strength: