linuxkpi: Add orderly_poweroff
This simply poweroff the system.
Needed by drm-kmod v5.8
Reviewed by: bz
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D34287
(cherry picked from commit 8021ba67231de90f17a951497474ab017b7dab9e)