Add a model/fixup string "lenovo-dock", for Thinkpad T431s, to allow sound in docking station.

Tested on Lenovo T431s with ThinkPad Mini Dock Plus Series 3
Signed-off-by: Ebben Aries <earies@dscp.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>