watchdog: New watchdog driver for MEN A21 watchdogs

This patch adds the driver for the watchdog devices found on MEN Mikro
Elektronik A21 VMEbus CPU Carrier Boards. It has DT-support and uses the
watchdog framework.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@men.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>