iio: imu: st_lsm6dsx: add event configurability on a per axis basis

iio: imu: st_lsm6dsx: add event configurability on a per axis basis

In order to be able to configure event detection on a per axis
basis (for either setting an event threshold/sensitivity value, or
enabling/disabling event detection), add new axis-specific fields
to struct st_lsm6dsx_event_src, and modify the logic that handles
event configuration to properly handle axis-specific settings when
supported by a given event source.
A future commit will add actual event sources with per-axis
configurability.

Signed-off-by: Francesco Lavra <flavra@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>