i2c: i801: Documentation update

The i2c-i801 driver documentation needs some dusting:
* Mention disable_features flag 0x20.
* The i2c_ec driver has been removed from the kernel long ago. Driver
  i2c-scmi serves the same purpose for more recent hardware.
* Replace obsolete /proc paths with equivalent /sys paths.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>