dt: watchdog: Add DT binding documentation for jz4740 watchdog timer

Add binding for jz4740 watchdog timer. It is a simple watchdog timer.

Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>