dt-bindings: timer: renesas,tmu: Document missing Gen3 SoCs

Add missing bindings for Gen3 SoCs.

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/20210211143102.350719-1-niklas.soderlund+renesas@ragnatech.se