iio: cros_ec: Expose hwfifo_timeout

Expose EC minimal interrupt period through buffer/hwfifo_timeout:
- Maximal timeout is limited to 65s.
- When timeout for all sensors is set to 0, EC will not send events,
  even if the sensor sampling rate is greater than 0.

Rename frequency to sampling_frequency to match IIO ABI.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>