pci: give preference to non-PCI_REGION_SYS_MEMORY regions when matching

When we search for an address match in pci_hose_{phys_to_bus,bus_to_phys}
we should give preference to memory regions that aren't system memory.

Its possible that we have over mapped system memory in the regions and
we want to avoid depending on the order of the regions.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>