Documentation: i2c: add i2c-sysfs into index

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

Fixes: 31df719 ("Documentation: i2c: Add doc for I2C sysfs")
Signed-off-by: Hu Haowen <[email protected]>
Signed-off-by: Wolfram Sang <[email protected]>

Author: Hu Haowen

Documentation/i2c/index.rst

@@ -17,6 +17,7 @@ Introduction
    busses/index
    i2c-topology
    muxes/i2c-mux-gpio
+   i2c-sysfs
 
 Writing device drivers
 ======================