docs/zh_CN: add riscv to zh_CN index

All the files in the riscv directory have been translated into
Chinese and it is time to add them to the zh_CN index.

Signed-off-by: Yanteng Si <[e-mail]>
Reviewed-by: Alex Shi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/translations/zh_CN/index.rst

···
    cpu-freq/index
    mips/index
    iio/index
+   riscv/index
 
 目录和表格
 ----------