bridges, the EBus bridge has resource ranges it claims exclusively to
map its children into in its BARs. Hence, we need to allocate these
completely and manage them for the children, instead of just passing
allocations through to the PCI layer as we did before.
While being there, split ebus_probe(), which did also contain code
normally belonging into the attach method, into ebus_probe() and
ebus_attach(), and perform some minor cleanups.
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