kernel/reboot: Add do_kernel_power_off()

Add do_kernel_power_off() helper that will remove open-coded pm_power_off
invocations from the architecture code. This is the first step on the way
to remove the global pm_power_off variable, which will allow us to
implement consistent power-off chaining support.

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