dt-bindings: thermal: rcar-gen3-thermal: Add r8a77961 support

Document R-Car M3-W+ (R8A77961) SoC bindings.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200306105503.24267-2-geert+renesas@glider.be