sound: Include sound(4) channel information in sndstat nvlist

Extend SNDST_DSPS_PROVIDER_INFO for sound(4) to include information
about each channel in a given device, similar to how cat'ing
/dev/sndstat with hw.snd.verbose=2 works.

While here, document all provider_info fields.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D45501