asus-laptop: add light sensor support

/proc/acpi/asus/lslvl is now /sys/.../asus-laptop/ls_level
/proc/acpi/asus/lssw is now /sys/.../asus-laptop/ls_switch nothing
else ..

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Len Brown <len.brown@intel.com>