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

Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/Documentation/sound/alsa/HD-Audio-Models.txt b/Documentation/sound/alsa/HD-Audio-Models.txt
index 5efacf0..1d38b0d 100644
--- a/Documentation/sound/alsa/HD-Audio-Models.txt
+++ b/Documentation/sound/alsa/HD-Audio-Models.txt
@@ -125,6 +125,7 @@
   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