ia64/PCI: register busn_res for root buses

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

[bhelgaas: changelog]
CC: Tony Luck <tony.luck@intel.com>
CC: Fenghua Yu <fenghua.yu@intel.com>
CC: linux-ia64@vger.kernel.org
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>