powerpc/83xx: merge PCI bridge additions

Nearly all mpc83xx-based boards have a common piece of code - one that
loops over all pci/pcie bridges and registers them. Merge that code into
a special function common to all boards.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>