[ALSA] hda-codec - Add support of ALC268 codec

Added the support of new ALC268 codec chip.
Signed-off-by: Kailang Yang <kailang@realtek.com.tw>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>

Documentation/sound/alsa/ALSA-Configuration.txt

@@ -822,6 +822,10 @@ Prior to version 0.9.0rc4 options had a 'snd_' prefix. This was removed.
 	  basic		fixed pin assignment w/o SPDIF
 	  auto		auto-config reading BIOS (default)
 
+	ALC268
+	  3stack	3-stack model
+	  auto		auto-config reading BIOS (default)
+
 	ALC882/885
 	  3stack-dig	3-jack with SPDIF I/O
 	  6stack-dig	6-jack digital with SPDIF I/O