iio: cros_ec: add 'id' sysfs entry

This new sysfs entry is used to interpret ring buffer information,
mainly by Android sensor HAL.
It expand to all sensors, the documentation about 'id' we can found
in Documentation/ABI/testing/sysfs-bus-iio-cros-ec.

Also fix typo in docs, I replace 'Septembre' by 'September'.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Fabien Lahoudere <fabien.lahoudere@collabora.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>