docs/zh_CN: add core-api boot-time-mm translation

Translate Documentation/core-api/boot-time-mm.rst into Chinese.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Link: https://lore.kernel.org/r/d802b8d0163db57ad326f512eaa48ea3c9e44814.1631846923.git.siyanteng@loongson.cn
Signed-off-by: Jonathan Corbet <corbet@lwn.net>