ARM/PCI: Set MPS before pci_bus_add_devices()

The MPS configuration should be done *before* pci_bus_add_devices().  After
pci_bus_add_devices(), drivers may be bound to devices, and the PCI core
shouldn't touch device configuration while a driver owns the device.

[bhelgaas: remove check for bus being non-NULL]
Reported-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>