dt-bindings: watchdog: update bindings for MT7629 SoC

This updates dt-binding documentation for MT7629 SoC

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>