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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring <[e-mail]>

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

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