LinuxKPI: add linux/pm_qos.h

Add a linux/pm_qos.h with three dummy functions and a struct as needed
by a driver and drm-kmod [1] with no intend to support this for the moment.

Submitted by: wulf (drm-kmod bits) [1]
Sponsored by: The FreeBSD Foundation (drm-kmod requested updates)
MFC after: 3 days
Reviewed by: hselasky (earlier version), wulf
Differential Revision: https://reviews.freebsd.org/D34234