PCI: add helpers for building PCI bus resource lists

We'd like to supply a list of resources when we create a new PCI bus,
e.g., the root bus under a PCI host bridge.  These are helpers for
constructing that list.

These are exported because the plan is to replace this exported interface:
    pci_scan_bus_parented()
with this one:
    pci_add_resource(resources, ...)
    pci_scan_root_bus(..., resources)

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>