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.org
Signed-off-by: Rob Herring <robh@kernel.org>