[PPC440SPe] Improve PCIe configuration space access

- correct configuration space mapping
- correct bus numbering
- better access to config space

Prior to this patch, the 440SPe host/PCIe bridge was able to configure only the
first device on the first bus. We now allow to configure up to 16 buses;
also, scanning for devices behind the PCIe-PCIe bridge is supported, so
peripheral devices farther in hierarchy can be identified.

Signed-off-by: Grzegorz Bernacki <gjb@semihalf.com>