stm class: p_sys-t: Document the configfs interface

This adds ABI documentation for the new configfs attributes that come
with the MIPI SyS-T protocol driver.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Tested-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>