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: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220719215134.1877363-1-robh@kernel.org


Signed-off-by: Mark Brown <broonie@kernel.org>