Provided a bus_space implementation for FDT, modelled on
bus_space_generic, but with a local version of the map address
routine that does a P->V translation, as is the case with NLM's
similar routine for XLP. It's not clear to me that this is the
right solution -- possibly this belongs in simplebus -- however,
it is sufficient to get the DE4 LED driver working.
Sponsored by: DARPA, AFRL