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