ALSA: hda: add mixers for analog mixer on 92hd75xx codecs

Add support for mixers on the analog mixer on some 92hd75xx codecs,
along with adding a 'Mixer' entry for it's connection on the dmux.

Signed-off-by: Matthew Ranostay <mranostay@embeddedalley.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>