platform/x86: Add driver for Apple gmux device

Apple laptops with hybrid graphics have a device named gmux that
controls the muxing of the LVDS panel between the GPUs as well as screen
brightness. This driver adds support for the gmux device. Only backlight
control is supported initially.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>
Tested-by: Grant Likely <grant.likely@secretlab.ca>