arm/PCI: remove arch pci_flags definition

The PCI core provides a pci_flags definition (currently __weak), so drop
the arm definition in favor of that.

We EXPORT_SYMBOL(pci_flags) as arm did previously.  I'm dubious about
this: no other architecture exports it, and I didn't see any modules in
the tree that reference it.

CC: Rob Herring <rob.herring@calxeda.com>
CC: Russell King <linux@arm.linux.org.uk>
CC: linux-arm-kernel@lists.infradead.org
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>