watchdog: imgpdc: Add reboot support

Register a restart handler that will restart the system by writing
to the watchdog's SOFT_RESET register.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Reviewed-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Tested-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Cc: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>