X86 drivers: Introduce msi-wmi driver

This driver serves backlight (including switching) and volume up/down
keys for MSI machines providing a specific wmi interface:
551A1F84-FBDD-4125-91DB-3EA8F44F1D45
B6F3EEF2-3D2F-49DC-9DE3-85BCE18C62F2

Signed-off-by: Thomas Renninger <trenn@suse.de>
CC: Carlos Corbacho <carlos@strangeworlds.co.uk>
CC: Matthew Garrett <mjg59@srcf.ucam.org>
Tested-by: Matt Chen <machen@novell.com>
Reviewed-by: Anisse Astier <anisse@astier.eu>
Signed-off-by: Len Brown <len.brown@intel.com>