PCI: add busn_res in struct pci_bus

This adds a busn_res resource in struct pci_bus.  This will replace the
secondary/subordinate members and will be used to build a bus number
resource tree to help with bus number allocation.

[bhelgaas: changelog]
CC: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>