PCI: add generic pci_hp_add_bridge()

This creates a generic pci_hp_add_bridge() that can be used by several
hotplug drivers.

[bhelgaas: split out from pciehp patch]
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>