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

Documentation/translations/zh_CN/devicetree/index.rst

@@ -22,9 +22,11 @@ Open Firmware 和 Devicetree
 .. toctree::
    :maxdepth: 1
 
+   usage-model
+
 Todolist:
 
-*   usage-model
+
 *   of_unittest
 *   kernel-api