ALSA: hda - Add missing description in HD-Audio-Models.txt

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

Documentation/sound/alsa/HD-Audio-Models.txt

@@ -125,6 +125,7 @@ ALC882/883/885/888/889
   macpro	MacPro support
   mb5		Macbook 5,1
   macmini3	Macmini 3,1
+  mba21		Macbook Air 2,1
   mbp3		Macbook Pro rev3
   imac24	iMac 24'' with jack detection
   imac91	iMac 9,1