In former commits, ALSA dice driver can handle available isochronous
streams. This commit adds support for several PCM substreams on the
streams.

The additional PCM substreams are available via another ALSA PCM character
devices so that one ALSA PCM application can handle them without cumbersome
operations. For example, two PCM substreams are available on each stream,
two ALSA character devices are added for them. In configuration space of
alsa-lib, it's represented with 'hw:0,0' and 'hw:0,1'.

The PCM substreams are constraint to parameters of the corresponding
streams. If the PCM substreams are unavailable for some reasons,
open(2) to ALSA PCM character device returns error and reports ENXIO.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>