ARM/PCI: 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 ARM bios32 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: fold in warning fix from Arnd Bergmann <arnd@arndb.de>:
http://lkml.kernel.org/r/20170621215323.3921382-1-arnd@arndb.de]
[bhelgaas: set bridge->ops for mv78xx0]
[bhelgaas: fold in fixes from Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>:
http://lkml.kernel.org/r/20170701135457.GB8977@red-moon

]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Andrew Lunn <andrew@lunn.ch>