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

Convert the Renesas R-Mobile/R-Car Timer Unit (TMU) Device Tree binding
documentation to json-schema.

Document missing properties.
Update the example to match reality.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20201110162014.3290109-3-geert+renesas@glider.be