PCI: versatile: 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 versatile host code to pci_scan_root_bus_bridge() to improve
the PCI root bus scanning interface.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
[bhelgaas: folded in fix from Arnd Bergmann <arnd@arndb.de>:
http://lkml.kernel.org/r/20170621215323.3921382-3-arnd@arndb.de

]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Rob Herring <robh@kernel.org>