Documentation/devicetree: Update PCI Device Tree bindings

Update broken links to PCI bus and interrupt mapping bindings.
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Signed-off-by: NRob Herring <robh@kernel.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
+http://www.firmware.org/1275/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
+http://www.firmware.org/1275/practice/imap/imap0_9d.pdf
 
 Additionally to the properties specified in the above standards a host bridge
 driver implementation may support the following properties: