[ALSA] hda-codec - Add 6stack model for ALC880

Documentation,HDA Codec driver
- Added a new '6stack' model for ALC880.
- Fixed the typo in 6stack-digout model name.
- Added description for missing models in ALSA-Configuration.txt.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
2 files changed