from where i sit, some of these questions and issues are the same as
need to be solved in the UltraSPARC `psycho' driver.  in the device
tree as seen from OFW, there are two separate `SUNW,psycho' devices
on the "mainbus".  but these share registers and other resources,
and thus something needs to be done such that they can sanely
attached, without resorting to some hack in the attach routine...