improved handling of mapping of devices who's registers do not reside on

improved handling of mapping of devices who's registers do not reside on
page boundaries:
 - change bus_tmp() to include the offset from the start of page in the
	returned KVA [rather than forcing each driver to add it back in
	individually]
 - changed bus_map() to include the offset from the start of page in the
	the returned value if a mapping is found in the PROM's KVA area
 - clarified a few comments