regulator: da9121: Use additionalProperties

There's no $ref at the top level so use additionalProperties instead of
unevaluatedProperties.

This fixes the following warning with the latest dtschema:

 Documentation/devicetree/bindings/regulator/dlg,da9121.yaml:
 'additionalProperties' is a required property

Reported-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Link: https://lore.kernel.org/r/20201109085438.16230-1-vincent.whitchurch@axis.com
Signed-off-by: Mark Brown <broonie@kernel.org>