watchdog: gpio_wdt: use core reboot notifier

Get rid of the custom reboot notifier block registration and use the one
provided by the watchdog core.

Note that this watchdog used to stop unconditionnaly on SYS_HALT and
SYS_POWER_OFF. The core function now calls ops->stop on SYS_HALT and
SYS_DOWN. To prevent the watchdog from being stopped on reboot, the
"always-running" property must be set, otherwise it will now be stopped.

Signed-off-by: Damien Riegel <damien.riegel@savoirfairelinux.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirlinux.com>
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>