docs: dt: usage_model.rst: fix link for DT usage

The devicetree.org doesn't host the Device_Tree_Usage page
anymore. So, fix the link to point to a new address.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>