mpc8640: Update the io_sel fields for PCI Express

Previously io_sel=0xe incorrect stated PCIE1 was enabled.  Also add
support for the mpc8640's PCIE2 interface.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>