powerpc/fsl/pci: Fix PCIe fixup regression

Upstream changes to the way PHB resources are registered
broke the resource fixup for FSL boards.

We can no longer rely on the resource pointer array for the PHB's
pci_bus structure, so let's leave it alone and go straight for
the PHB resources instead. This also makes the code generally
more readable.

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