MFC r338422: Adding support for CS46xx MIDI output.

With this patch, users can play the MIDI files through /dev/sequencer
device with tools like playmidi. The audio output will go through
the external MIDI device such like wavetable synthesis card.

Reviewed by: matk (a long time ago), kib
Tested with: Terratec SiXPack 5.1+ + Yamaha DB50XG