watchdog: jz4740: Add support for the watchdog in jz4780 SoC

The watchdog unit present in the JZ4780 works the same as the one in the
JZ4740.

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>