powerpc/PCI: Fix lookup of linux,pci-probe-only property

When find_and_init_phbs() looks for the probe-only property, it seems to
trust the firmware to be correctly written, and assumes that there is a
parameter to the property.

It is conceivable that the firmware could not be that perfect, and it could
expose this property naked (at least one arm64 platform seems to exhibit
this exact behaviour).  The setup code the ends up making a decision based
on whatever the property pointer points to, which is likely to be junk.

Instead, switch to the common of_pci.c implementation that doesn't suffer
from this problem and ignore the property if the firmware couldn't make up
its mind.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>