[ALSA] hda: Mono mux mixer support

Add support for the mono mux on several 92HD7xxx codecs.
Creates a dynamic mixer for the mux selection.

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