x86/PCI: put busn resource in pci_root_info for native host bridge drivers

Add the host bridge bus number aperture to the resource list.
Like the MMIO and I/O port apertures, this will be used when assigning
resources to hot-added devices or in the case of conflicts.

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