This adds get_pci_domain_busn_res(), which returns the root of the
bus number resource tree for a domain, creating it if necessary.
We will later populate the tree with the bus numbers used by host
bridges and P2P bridges in the domain.

[bhelgaas: changelog]
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>