microblaze/PCI: use pci_scan_root_bus()

Microblaze doesn't need to replace pci_scan_child_bus() or do anything
special before pci_bus_add_devices(), so we can use the more generic PCI
path in pci_scan_root_bus().

CC: Michal Simek <monstr@monstr.eu>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>