watchdog: mach-moxart: add restart handler

mach-moxart lacks a separate register for reset; as a workaround,
add a function that can be hooked to arm_pm_restart.

Signed-off-by: Jonas Jensen <jonas.jensen@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>