ALSA: hda - Split ALC268 acer model

There are actually two variants of ALC268 Acer implementation, one
with an analog built-in mic (pin 0x19) and another with a digital
mic (pin 0x12).  Created a new model, acer-dmic, for the latter case
now.

So far, all known models are assigned to be analog-mic, according to
the BIOS setup.  If this doesn't match with the actual case, one needs
to try model=acer-dmic, and fix the entry to point ALC268_ACER_DMIC
if it works.

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

Documentation/sound/alsa/ALSA-Configuration.txt

@@ -844,6 +844,7 @@ Prior to version 0.9.0rc4 options had a 'snd_' prefix. This was removed.
 	  3stack	3-stack model
 	  toshiba	Toshiba A205
 	  acer		Acer laptops
+	  acer-dmic	Acer laptops with digital-mic
 	  acer-aspire	Acer Aspire One
 	  dell		Dell OEM laptops (Vostro 1200)
 	  zepto		Zepto laptops