docs: dt: convert usage-model.txt to ReST

- Add a SPDX header;
- Adjust document title;
- Use footnoote markups;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add it to devicetree/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>