docs/zh_CN: add iio ep93xx_adc.rst translation

This patch translates Documentation/iio/ep93xx_adc.rst into Chinese.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com>
Reviewed-by: Huacai Chen <chenhuacai@kernel.org>
Link: https://lore.kernel.org/r/20210121033302.558935-2-siyanteng@loongson.cn


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