dt-bindings: timer: renesas: cmt: Convert to json-schema

Convert the Renesas Compare Match Timer (CMT) Device Tree binding
documentation to json-schema.

Document missing properties.
Update the example to match reality.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>