Documentation/devicetree: add a small note on PCI

Throw in a file with references to the IEEE binding documents.
Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/pci/pci.txt

+PCI bus bridges have standardized Device Tree bindings:
+
+PCI Bus Binding to: IEEE Std 1275-1994
+http://www.openfirmware.org/ofwg/bindings/pci/pci2_1.pdf
+
+And for the interrupt mapping part:
+
+Open Firmware Recommended Practice: Interrupt Mapping
+http://www.openfirmware.org/1275/practice/imap/imap0_9d.pdf