dt-bindings: timer: renesas,cmt: Document R8A77961

Add missing bindings for M3-W+.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210211143344.352588-1-niklas.soderlund+renesas@ragnatech.se