powerpc/4xx: Enable PCI domains on 4xx

4xx chips commonly now have multiple PHBs, there is no reason to not
enable PCI domains on them. The main issue with PCI domains is X but
currently its already somewhat busted for other reasons such as the
36-bit physical address space, which I'm fixing separately.

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