dell-laptop: create a platform device as a parent for the rfkill devices etc.

dell-laptop may not need to export any sysfs files, but it should still
create a platform device as a parent for the rfkill and backlight
devices.  Otherwise sysfs will display these as "virtual" devices,
with no connection to either physical hardware or the dell-laptop
module.

Apparently this is useful for hardware detection.

Signed-off-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Acked-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Len Brown <len.brown@intel.com>