pci/of: Consolidate pci_device_to_OF_node()

All archs do more or less the same thing now, move it into
a single generic place.

I chose pci.h rather than of_pci.h to avoid having to change
all call-sites to include the later.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Michal Simek <monstr@monstr.eu>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>