backlight: Add Nvidia-based Apple Macbook Pro backlight driver

Nvidia-based Apple Macbook Pros don't appear to handle backlight control
through the graphics card registers or ACPI, but instead trigger changes
via SMI calls. This driver registers a generic backlight device that
lets existing userspace deal with it. Code derived from Julien Blache's
Pommed application.

Signed-off-by: Julien Blache <jb@jblache.org>
Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Richard Purdie <rpurdie@rpsys.net>