bus memory and I/O space access functions/macros, to be used by
machine-independent code for more sane access to bus resources.
New functions will be added to this set, in the future, as appropriate,
but this is a good starting set. Defines:
bus_{io,mem}_{map,unmap}
bus_{io,mem}_{read,write}_{1,2,4,8}
functions, and several types to go with them.
jonathan