powerpc/fsl_pci: Don't set up inbound windows in kdump crash kernel

Otherwise, because the top end of the crash kernel is treated as the
absolute top of memory rather than the beginning of a reserved region,
in-flight DMA from the previous kernel that targets areas above the
crash kernel can trigger a storm of PCI errors.  We only do this for
kdump, not normal kexec, in case kexec is being used to upgrade to a
kernel that wants a different inbound memory map.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: Mingkai Hu <Mingkai.hu@freescale.com>