Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Enrich with some register descriptions and additional register macros.

Obtained from: OpenBSD

Implement interrupt resource allocation and setup. Set the interrupt
group number properly based on the board id. Perform dummy reads of
registers after writing to flush the hardware write buffers.

This gets the soon to be committed zs attachment working.

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.