mpc7448hpc2: Fix PCI I/O space mapping.

PCI I/O space is currently mapped 1:1 at 0xFA000000. Linux requires
PCI I/O space to start at 0 on the PCI bus. This patch maps PCI I/O
space such that 0xFA000000 in the processor's address space maps to 0
on the PCI I/O bus.

Signed-off-by Randy Vinson <rvinson@mvista.com>
Acked-by: Roy Zang <tie-fei.zang@freescale.com>