alienware-wmi: Add initial support for alienware graphics amplifier.

The alienware graphics amplifier is a device that provides external access
to a full PCIe slot, USB hub, and additional control zone.

This patch enables support for reading status whether the cable is plugged
in as well as for setting the colors in the new zone on the amplifier.

Signed-off-by: Mario Limonciello <mario_limonciello@dell.com>
[dvhart: minor comment formatting fixes]
Signed-off-by: Darren Hart <dvhart@linux.intel.com>