watchdog: da9063: Add restart handler support

Register a restart handler for the da9063 watchdog.  System restart is
triggered by sending the shutdown command to the PMIC.
As more-suitable restart handlers may exist, the priority of the
watchdog restart handler is set to 128.

The actual restart method was inspired by a platform-specific patch from
the BSP by Hisashi Nakamura <hisashi.nakamura.ak@renesas.com>.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Steve Twiss <stwiss.opensource@diasemi.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>