docs/zh_CN: add core-api printk-formats.rst translation

This patch translates Documentation/core-api/printk-formats.rst into Chinese.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Wu XiangCheng <bobwxc@email.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Reviewed-by: Jiaxun Yang <jaixun.yang@flygoat.com>
Link: https://lore.kernel.org/r/b9dea1426e43d8848f1a8b7319c002418aebd3df.1621239725.git.siyanteng@loongson.cn


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