Add input device support to the hotkey subdriver.

Hot keys that have a valid keycode mapping are reported through the input
layer if the input device is open.  Otherwise, they will be reported as
ACPI events, as they were before.

Scan codes are reported (using EV_MSC MSC_SCAN events) along with EV_KEY
KEY_UNKNOWN events.

For backwards compatibility purposes, hot keys that used to be reported
through ACPI events are not mapped to anything meaningful by default.
Userspace is supposed to remap them if it wants to use the input device for
hot key reporting.

This patch is based on a patch by Richard Hughes <hughsient@gmail.com>.
Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Richard Hughes <hughsient@gmail.com>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Len Brown <len.brown@intel.com>