ALSA: hda: slave digital out support

Added support for playing a stream on multiple digital outs. This is done
by defining codec->slave_dig_outs as array of hda_nid_t with a null-terminated entry to set the
slave SPDIF outs, in which the slave outs have cloned settings of the master out (e.g. dig_out_nid).

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