[POWERPC] 4xx: PLB to PCI-X support

This adds base support code for the 4xx PCI-X bridge. It also provides
placeholders for the PCI and PCI-E version but they aren't supported
with this patch.

The bridges are configured based on device-tree properties.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>