dt-bindings: watchdog: renesas,wdt: Document RZ/V2N (R9A09G056) support

Document support for the watchdog IP found on the Renesas RZ/V2N
(R9A09G056) SoC. The watchdog IP is identical to that on RZ/V2H(P),
so `renesas,r9a09g057-wdt` will be used as a fallback compatible,
enabling reuse of the existing driver without changes.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20250502120054.47323-1-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>