PCI: xgene: Convert PCI scan API to pci_scan_root_bus_bridge()

PCI: xgene: Convert PCI scan API to pci_scan_root_bus_bridge()

The introduction of pci_scan_root_bus_bridge() provides a PCI core API to
scan a PCI root bus backed by an already initialized struct pci_host_bridge
object, which simplifies the bus scan interface and makes the PCI scan root
bus interface easier to generalize as members are added to the struct
pci_host_bridge.

Convert PCI xgene host code to pci_scan_root_bus_bridge() to improve the
PCI root bus scanning interface.

Tested-by: Khuong Dinh <kdinh@apm.com>	# with e1000e
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Tanmay Inamdar <tinamdar@apm.com>