kernel/reboot: Change registration order of legacy power-off handler

We're unconditionally registering sys-off handler for the legacy
pm_power_off() callback, this causes problem for platforms that don't
use power-off handlers at all and should be halted. Now reboot syscall
assumes that there is a power-off handler installed and tries to power
off system instead of halting it.

To fix the trouble, move the handler's registration to the reboot syscall
and check the pm_power_off() presence.

Fixes: 0e2110d2 ("kernel/reboot: Add kernel_can_power_off()")
Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>