[ALSA] Added model for Uniwill laptop with ALC861

Added a new model 'uniwill-m31' for Uniwill laptops with ALC861
codec chip.  The patch is taken from ALSA bug#2035, and modifeid.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>