MFC: r182688

Bypass isa_probe_children(9) and directly call bus_generic_attach(9)
in order to avoid the invasive probes done by identify-routines of
ISA drivers, which may access unassigned addresses or those of
unrelated devices and thus in turn can trigger master/target aborts
as revealed by r182108 and ahc(4). I think that this is also the
cause of the hang previously seen on B100 blades during boot.
Bypassing isa_probe_children(9) also avoids adding ISA hints, which
just can be wrong for sparc64.

Approved by: re (kib)