ymf278b: Use the memory system to access wavetable data. This should make it
  possible to hook up RAM as well as ROM to the device, e.g. for computer
  sound cards in MESS.

psikyo4: Improve and clean up wavetable ROM banking. The mask ROM tests in
  hotgm4ev and hotgmcki pass now. loderndf still fails for unknown reasons.
  Miscellaneous cleanups as well. [Alex Jackson]

(nw) I'm using the multipcm workaround for default address maps for now.
The core change I discussed will go in a separate commit, in case it causes
more problems than anticipated.