dt-bindings: Update schema project location to devicetree.org github group

The DT schema tools are moving from my personal GH repo to the
devicetree.org group on GH. The new location is here:

https://github.com/devicetree-org/dt-schema.git



The old repo will be kept as a mirror.

Signed-off-by: Rob Herring <robh@kernel.org>