MPC8555(E) has 8 LAWs, so don't make that the default case. Current
processors have 12 LAWs so use that as the default instead.
o Determine the target ID of the PCI/PCI-X and PCI-E controllers in
a way that's more future proof. There's almost a perfect mapping
from HC register offset to target ID, so use that as the default.
Handle the MPC8548(E) specially, since it has a non-standard target
ID for the PCI-E controller. Don't worry about whether the processor
implements the target ID here, because we should not get called for
PCI/PCI-X or PCI-E host controllers that don't exist.
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