dt-bindings: watchdog: renesas-wdt: Document r8a77961 support

Document support for the Watchdog Timer in the Renesas R-Car M3-W+
(R8A77961) SoC.

No driver update is needed.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Rob Herring <robh@kernel.org>