docs/zh_CN: add devicetree usage-model translation

Translate .../Devicetree/usage-model.rst into Chinese.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/abc4bfa5a8eb438013913bf8ea5f8a29e5730f33.1646642188.git.siyanteng@loongson.cn


Signed-off-by: Jonathan Corbet <corbet@lwn.net>