ACPI: thinkpad-acpi: register input device

Register an input device to send input events to userspace.

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>