kernel/reboot: Add stub for pm_power_off

Add weak stub for the global pm_power_off callback variable. This will
allow us to remove pm_power_off definitions from arch/ code and transition
to the new sys-off based API that will replace the global variable.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>