dt-bindings: watchdog: renesas-wdt: Document r8a774b1 support

RZ/G2N (a.k.a. R8A774B1) watchdog implementation is compatible
with R-Car Gen3, therefore add the relevant documentation.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>