linuxkpi: Add pwm.h

Provide dummy functions needed for drm-kmod.

Reviewed by: bz
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D35572

(cherry picked from commit 2b743f65f1c9352686ba0396f8ca3914a8025c4a)