Add drivers for the central and fhc busses found in enterprise class

Add drivers for the central and fhc busses found in enterprise class
UltraSPARCs, and an eeprom attachment for fhc, which allows the date
to be set properly on these machines.  Central is a wierd bus which
seems to only ever have 1 fhc attached to it.   FHC (FireHose Controller)
is another wierd bus with various things on it depending where its attached.
The fhc attached to central has eeprom and zs, and the fhcs which attach
directly to nexus have simm-status, environment and other nodes, none of
which I'll probably ever have documentation for.

Thanks to Ade Lovett for providing access to an 8 cpu e4500.