[ALSA] hda-codec - Add the support of Dell OEM laptops with ALC268

Added the support of Dell OEM laptops (Vostro 1200) with ALC268 codec.
The new model=dell is provided.

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