Clean up the name space here a bit.
Add a `busname' element to struct ofprobe (now struct ofbus_attach_args), and
check against it.
Nuke `ofroot'.
Rename the `openfirm' attribute to `ofbus'.
The old ofbprint() wasn't particularly good (it forced the root node
to have a 'name' property, which apparently violates the OFW spec), and
the new one (though it normally prints more useful information) has
nothing useful to say about the root node. It's not clear to me that
'ofroot' should exist at all.
