ALSA: hda - Add documentation for CS4208 fixups

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

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

@@ -296,6 +296,12 @@ Cirrus Logic CS4206/4207
   imac27	IMac 27 Inch
   auto		BIOS setup (default)
 
+Cirrus Logic CS4208
+===================
+  mba6		MacBook Air 6,1 and 6,2
+  gpio0		Enable GPIO 0 amp
+  auto		BIOS setup (default)
+
 VIA VT17xx/VT18xx/VT20xx
 ========================
   auto		BIOS setup (default)