[ALSA] hda-codec - Add support for Gigabyte S-Series GA-M57SLI-S4 motherboard

Added the support for Gigabyte S-Series GA-M57SLI-S4 motherboard
(model=6stack-dig).

Signed-off-by: Tobin Davis <tdavis@dsl-only.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>