LinuxKPI: pm.h: add dummy pm_wakeup_event()

Add a dummy implementation of pm_wakeup_event() which is used to notify
the power management system about a wakeup (which we currently do not
implement yet).

Sponsored by: The FreeBSD Foundation
Approved by: re (cperciva)
Differential Revision: https://reviews.freebsd.org/D38239

(cherry picked from commit 934cb7a3316609380fdea8da73233759f6dfff43)
(cherry picked from commit fbc5dea2a3af8edf1f22603605f78fe9f9b71494)