samsung-laptop: use a sysfs group

Will be usefull later when we will have more platform sysfs files
like battery_life_extender or usb_charge.

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Matthew Garrett <mjg@redhat.com>