PCI: xgene: Assign resources to bus before adding new devices

The X-Gene PCIe driver assumes pci_scan_root_bus() assigns resources as
proposed in [1].  But we dropped patch [1] because it would break some
architectures, which means the X-Gene PCIe driver is currently broken.

Add calls to scan the bus, assign resources, and add devices in the X-Gene
driver to fix this.

[bhelgaas: changelog]
[1] http://lkml.kernel.org/r/1412000971-9242-11-git-send-email-Liviu.Dudau@arm.com
Signed-off-by: Duc Dang <dhdang@apm.com>
Signed-off-by: Tanmay Inamdar <tinamdar@apm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>