docs/zh_CN: add devicetree of_unittest translation

Translate .../Devicetree/of_unittest.rst into Chinese.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/da75225df2ac7bee8de653d95b2a872510b34f7d.1646642188.git.siyanteng@loongson.cn


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