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>