As the power button causes a wake from suspend, we need to register
the event with the pm sustem to avoid racing with suspend.

As the input event is reported in the interrupt handler, as simple
pm_wakeup_event() is sufficient.
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>