powerpc/pci: Pull ppc32 PCI features into common

Some of the PCI features we have in ppc32 we will need on ppc64
platforms in the future.  These include support for:

* ppc_md.pci_exclude_device
* indirect config cycles
* early config cycles

We also simplified the logic in fake_pci_bus() to assume it will always
get a valid pci_controller.  Since all current callers seem to pass it
one.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>