PCI: Add pci_device_to_OF_node() stub for !CONFIG_OF

Add a stub for pci_device_to_OF_node() so drivers don't need to
use #ifdef CONFIG_OF around calls to it.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>