Documentation: i2c: add i2c-sysfs into index

Append i2c-sysfs to toctree in order to get rid of building warnings.

Fixes: 31df7195 ("Documentation: i2c: Add doc for I2C sysfs")
Signed-off-by: Hu Haowen <src.res@email.cn>
Signed-off-by: Wolfram Sang <wsa@kernel.org>