asus-laptop: platform dev as parent for led and backlight

Makes asus-laptop platform device the parent device of
backlight and led classes.

With this patch, leds and backlight are also available in
/sys/devices/platform/asus-laptop/ like thinkpad_acpi.

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