dt-bindings: watchdog: renesas,wdt: Document r8a774e1 support

dt-bindings: watchdog: renesas,wdt: Document r8a774e1 support

RZ/G2H (a.k.a. R8A774E1) watchdog implementation is compatible
with R-Car Gen3, therefore add the relevant documentation.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1594811350-14066-17-git-send-email-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>