powerpc/pci: Fix IMMRBAR address

For PEXCSRBAR, bit 3-0 indicate prefetchable and address type.
So when getting base address, these bits should be masked,
otherwise we may get incorrect base address.

Signed-off-by: Minghuan Lian <Minghuan.Lian@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>