eeepc-wmi: use a platform device as parent device of all sub-devices

Add a platform device and use it as the parent device of all sub-devices.

Signed-off-by: Yong Wang <yong.y.wang@intel.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>
Reviewed-by: Corentin Chary <corentincj@iksaif.net>