The code in rtemsbsd/freebsd/machine/bus.h assumed that all bus space
accesses were through memory in a simple fashion. The i386 has a true
distinction between I/O and memory space which must be accounted for.
This may not be the eventual structure of this code but we must
have different bus space accessors for different hardware configurations.
And in many, if not most, cases we will want these to be inlined for
performance.
