docs/zh_CN: add openrisc translation to zh_CN index

This patch add zh_CN/openrisc to zh_CN/index.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Wu XiangCheng <bobwxc@email.cn>
Link: https://lore.kernel.org/r/162fb50638028c9b0a92a0ce5c53e691be0cfec2.1618568135.git.siyanteng@loongson.cn


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