docs/zh_CN: add riscv vm-layout translation

Translate .../riscv/vm-layout.rst into Chinese.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/b59965f7f88ea22ec824dc2e92561b018fd370d1.1646108017.git.siyanteng@loongson.cn


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