PCI: introduce pci_assign_unassigned_bridge_resources

For use by pciehp.

pci_setup_bridge() will not check enabled for the slot bridge, otherwise
update res is not updated to bridge BAR.  That is, bridge is already
enabled for port service.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Reviewed-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>