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

When pci-host-generic looks for the probe-only property, it seems to trust
the DT to be correctly written, and assumes that there is a parameter to
the property.

Unfortunately, this is not always the case, and some firmware expose this
property naked.  The driver ends up making a decision based on whatever the
property pointer points to, which is likely to be junk.

Switch to the common of_pci.c implementation that doesn't suffer from this
problem.

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: Will Deacon <will.deacon@arm.com>