dt-bindings: mfd: st,stmfx: Remove extra additionalProperties

The following build error is seen with 'make dt_binding_check':

  CHKDT   Documentation/devicetree/bindings/mfd/st,stmfx.yaml
/home/fabio/linux-next/Documentation/devicetree/bindings/mfd/st,stmfx.yaml: properties:pinctrl:patternProperties: {'enum': ['$ref', 'additionalItems', 'additionalProperties', 'allOf', 'anyOf', 'const', 'contains', 'default', 'dependencies', 'deprecated', 'description', 'else', 'enum', 'if', 'items', 'maxItems', 'maximum', 'minItems', 'minimum', 'multipleOf', 'not', 'oneOf', 'pattern', 'patternProperties', 'properties', 'propertyNames', 'required', 'then', 'unevaluatedProperties']} is not allowed for 'additionalProperties'

Remove the extra 'additionalProperties' to pass the build.
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/st,stmfx.yaml

@@ -75,8 +75,6 @@ properties:
 
     additionalProperties: false
 
-    additionalProperties: false
-
     required:
       - compatible
       - "#gpio-cells"