acer-wmi: Add acer wmi hotkey events support

Add acer wmi hotkey event support. Install a wmi notify handler to
transfer wmi event key to key code, then send out keycode through acer
wmi input device to userland.

Signed-off-by: Lee, Chun-Yi <jlee@novell.com>
Acked-by: Dmitry Torokhov <dtor@mail.ru>
Acked-by: Thomas Renninger <trenn@suse.de>
Acked-by: Jiri Benc <jbenc@suse.cz>
Signed-off-by: Carlos Corbacho <carlos@strangeworlds.co.uk>
Cc: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Matthew Garrett <mjg@redhat.com>