[ALSA] hda-codec - Add test model for ALC268

This implements a test model for the ALC268.  It depends on the feature
added by alc260-test-eapd-0.2.diff.  This patch also adds a mention of
the ALC260 test model to ALSA-Configuration.txt since this seems to have
been missed.

Signed-off-by: Jonathan Woithe <jwoithe@physics.adelaide.edu.au>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>