samsung-laptop: Samsung R410P backlight driver

Here's a trivial patch which adds support to the backlight device found
in Samsung R410 Plus laptops.

Signed-off-by: Alberto Mardegan <mardy@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
[mmarek: cherry-picked from staging commit d542f180

]
Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Matthew Garrett <mjg@redhat.com>