dt-bindings: input: syna,rmi4: Make "additionalProperties: true" explicit

Make it explicit that the not yet documented child nodes have additional
properties and the child node schema is not complete.
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230926144249.4053202-1-robh@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/input/syna,rmi4.yaml

@@ -164,6 +164,8 @@ patternProperties:
 
   "^rmi4-f[0-9a-f]+@[0-9a-f]+$":
     type: object
+    additionalProperties: true
+
     description:
       Other functions, not documented yet.