docs/zh_CN: add cpu-freq index.rst translation

This patch translates Documention/cpu-freq/index.rst into Chinese.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/20210406070239.19910-5-siyanteng@loongson.cn


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