iio: imu: st_lsm6dsx: make event_settings more generic

iio: imu: st_lsm6dsx: make event_settings more generic

The st_lsm6dsx_event_settings structure contains fields specific for one
event type (wakeup). In preparation for adding support for more event
types, introduce an event id enum and a generic event source structure, and
replace wakeup-specific data in struct st_lsm6dsx_event_settings with an
array of event source structures.

Signed-off-by: Francesco Lavra <flavra@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>