dell-wmi: new driver for hotkey control

Add a WMI driver for Dell laptops. Currently it does nothing but send a
generic input event when a button with a picture of a battery on it is
pressed, but maybe other uses will appear over time.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Cc: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>