samsung-laptop: add a small debugfs interface

This allow to call arbitrary sabi commands wihout
modifying the driver at all. For example, setting
the keyboard backlight brightness to 5 using debugfs
interface can be done like that:

 ; Set the command
 echo 0x78 > command
 ; Set the data
 echo 0x0582 > d0
 ; Fill the rest with 0
 echo 0 > d1
 echo 0 > d2
 echo 0 > d3
 ; And issue the command
 cat call

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