eeepc-wmi: add debugfs entries

eeepc-wmi/    - debugfs root directory
  dev_id      - current dev_id
  ctrl_param  - current ctrl_param
  devs        - call DEVS(dev_id, ctrl_param) and print result
  dsts        - call DSTS(dev_id)  and print result

DEVS and DSTS are the main functions used in eeepc-wmi, this
will allow to test new features without patching the drivers.

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Matthew Garrett <mjg@redhat.com>