eeepc-wmi: add an eeepc_wmi context structure

Add an eeepc_wmi context structure to manage all the sub-devices
that will be implemented later on. Put input device into it first.

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>