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
Approved by: re (kib)
Tested with: Terratec SiXPack 5.1+ + Yamaha DB50XG
MFC after: 4 weeks