asus-laptop: Add support for Keyboard backlight

Add support for keyboard backlight found in Asus U50VG.

The SMC driver for the Apples does it via LED. To be
consistent with that we create /sys/class/leds/asus::kbd_backlight/
to control the keyboard backlight.

SLKB and GLKB are used to get/set the backlight. On
the U50VG is supports 4 brightness level, but this may
change with other models.

SLKB take a 8 bit integer where the higher bit is used
to toggle the backlight, and the over 7 bits control the
brightness level.

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