dt-bindings: update OpenFirmware document links to devicetree.org

The mirrors for old, but still referenced OF documents have disappeared.
A new mirror has been setup on devicetree.org at:

http://devicetree.org/open-firmware/home.html

Update the URLs in the binding documents with the new mirror.

Cc: Brian Norris <briannorris@chromium.org>
Reviewed-by: Harvey Hunt <harvey.hunt@imgtec.com>
Signed-off-by: Rob Herring <robh@kernel.org>
diff --git a/Documentation/devicetree/booting-without-of.txt b/Documentation/devicetree/booting-without-of.txt
index fb74044..417f911 100644
--- a/Documentation/devicetree/booting-without-of.txt
+++ b/Documentation/devicetree/booting-without-of.txt
@@ -1282,7 +1282,7 @@
 The interrupt tree model is fully described in the
 document "Open Firmware Recommended Practice: Interrupt
 Mapping Version 0.9".  The document is available at:
-<http://www.openfirmware.org/ofwg/practice/>
+<http://www.devicetree.org/open-firmware/practice/>
 
 1) interrupts property
 ----------------------