platform/x86: add MXM WMI driver.

MXM is a laptop graphics card form-factor + interface specification,
this adds an initial stub driver to talk to the MXM WMI interface.

The only method used is the MUX switching method needed to do switchable
graphics on the nvidia chipsets.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Acked-by: Matthew Garrett <mjg@redhat.com>