microblaze/PCI: convert to pci_create_root_bus() for correct root bus resources

Convert from pci_create_bus() to pci_create_root_bus().  This way the root
bus resources are correct immediately.  This patch doesn't fix a problem
because microblaze fixed the resources before scanning the bus, but it makes
microblaze more consistent with other architectures.  It also allows us to
use the pci_scan_root_bus() path safely.

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