MIPS: PCI: Scan busses when they are registered

The patch below changes register_pci_controller() such that controllers
being added after pcibios_init() has run are be scanned immediately.

This is needed for example by the BCM47xx PCI controller, which is located
on the SSB bus, which is now initialized after the PCI subsystem.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>