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
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D38239

(cherry picked from commit 934cb7a3316609380fdea8da73233759f6dfff43)