watchdog: stmp3xxx: Stop the watchdog on system halt

This allows the system to actually halt even if userspace forgot to
disable the watchdog first. Old behaviour was that the watchdog forced
the system to boot again.

Signed-off-by: Harald Geyer <harald@ccbib.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>