thinkpad-acpi: add support for model-specific keymaps

Use the quirks engine to select model-specific keymaps, which makes
it much easier to extend should we need it.

Keycodes are based on the tables at
http://www.thinkwiki.org/wiki/Default_meanings_of_special_keys.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Matthew Garrett <mjg@redhat.com>