docs/zh_CN: core-api: Add timekeeping Chinese translation

Translate core-api/timekeeping.rst into Chinese.

Last English version used:

commit 3dc6ffae ("timekeeping: Introduce fast accessor to clock tai").

Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Reviewed-by: Wu XiangCheng <bobwxc@email.cn>
Link: https://lore.kernel.org/r/8d15ff60aaadd53065b3bc2da7d53a520cf01c53.1666171735.git.zhoubinbin@loongson.cn


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