PCI: add generic device into pci_host_bridge struct

Use that device for pci_root_bus bridge pointer.

Use pci_release_bus_bridge_dev() to release allocated pci_host_bridge in
remove path.

Use root bus bridge pointer to get host bridge pointer instead of searching
host bridge list.  That leaves the host bridge list unused, so remove it.

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