PCI: make pci_flags always available

If we move resource assignment functions into the core, we'll still
need a way for architectures to prevent reassignment, e.g., the
"pci_probe_only" functionality, and we'll need a generic, always
available way the core can test for that.  The "pci_flags"
arrangement used by several architectures seems like a convenient
way to do this.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>