powerpc/powernv: Fix PCI resource handling

Recent changes to the handling of PCI resources for host bridges
are breaking the PowerNV code for assigning resources on IODA.

The root of the problem is that the pci_bus attached to a host
bridge no longer has its "legacy" resource pointers populated
but only uses the newer list instead.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>