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

Convert the Renesas OS Timer (OSTM) Device Tree binding documentation to
json-schema.

Document missing properties.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Rob Herring <robh@kernel.org>