External events can cause the backlight level to change (AC adapter
plug/unplug for example) so cache the value there too.
PR: 257796
Sponsored by: Beckhoff Automation GmbH & Co. KG
MFC after: 1 week
This is a simple subsystem that allow drivers to register as a backlight.
Each backlight creates a device node under /dev/backlight/backlightX and
an alias based on the name provided.
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26250