dt-bindings: watchdog: renesas,wdt: Document RZ/G2UL SoC

Document RZ/G2UL WDT bindings. RZ/G2UL WDT is similar to one found
on the RZ/G2L SoC. No driver changes are required as generic compatible
string "renesas,rzg2l-wdt" will be used as a fallback.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20220424071323.151757-1-biju.das.jz@bp.renesas.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>