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: NRob Herring <robh@kernel.org>
Signed-off-by: NVincent Whitchurch <vincent.whitchurch@axis.com>
Link: https://lore.kernel.org/r/20201109085438.16230-1-vincent.whitchurch@axis.comSigned-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/dlg,da9121.yaml

@@ -22,7 +22,7 @@ properties:
     $ref: "regulator.yaml#"
     type: object
 
-unevaluatedProperties: false
+additionalProperties: false
 
 required:
   - compatible