[POWERPC] 4xx: PCIe: Increase max busses per port to 64

Because of how big mapping the config space is (1M per bus), we limit how
many busses we support for now. In the long run, we could replace that
with something akin to kmap_atomic instead.

This patch changes the limit from currently 16 to 64.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>