Expose the ACPI power button, sleep button and LID state as evdev's.

This allows libinput to disable touchpads when the lid is closed and
various desktop environments can show power-off dialogs when the power
button is pressed. While the latter is doable with devd a
cross-platform solution is nicer.

Submitted by: Greg V <greg@unrelenting.technology>
Differential Revision: https://reviews.freebsd.org/D23863
MFC after: 1 week
Sponsored by: Mellanox Technologies