Documentation: Add entry for dell-laptop sysfs interface

Add the documentation for the new sysfs interface of dell-laptop
that allows to configure the keyboard illumination on Dell systems.

Signed-off-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>