ALSA: hda: document VIA models

Add documentation about the autodetection of the VIA codec models to
avoid the false impression that they are not supported.
Signed-off-by: NClemens Ladisch <clemens@ladisch.de>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

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

@@ -416,3 +416,7 @@ Cirrus Logic CS4206/4207
   mbp55		MacBook Pro 5,5
   imac27	IMac 27 Inch
   auto		BIOS setup (default)
+
+VIA VT17xx/VT18xx/VT20xx
+========================
+  auto		BIOS setup (default)