dt-bindings: watchdog: renesas-wdt: Add support for the R8A77990 wdt

Document support for the Watchdog Timer (WDT) Controller in the Renesas
R-Car E3 (R8A77990) SoC.

No driver update is needed.

Signed-off-by: Masaharu Hayakawa <masaharu.hayakawa.ry@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>