of/pci: Add of_pci_check_probe_only to parse "linux,pci-probe-only"

Both pci-host-generic and Pseries parse the "linux,pci-probe-only" property
to engage the PCI_PROBE_ONLY mode, and both have a subtle bug that can be
triggered if the property has no parameter.

Provide a generic, safe implementation that can be used by both.

[bhelgaas: fold in #include <asm-generic/pci-bridge.h>]
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rob Herring <robh@kernel.org>