ALSA: hda - Support multiple digital outs with auto-probing

Added the support of multiple digital outputs via auto-probing for
Realtek ALC88x codecs.  The multiple outputs are handled as slave
streams, so only one PCM stream (and the corresponding IEC958*
elements) is provided to control both digital outputs.

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