iio: accel: adxl345: add activity event feature

iio: accel: adxl345: add activity event feature

Enable the sensor to detect activity and trigger interrupts accordingly.
Activity events are determined based on a threshold, which is initialized
to a sensible default during probe. This default value is adopted from the
legacy ADXL345 input driver to maintain consistent behavior.

The combination of activity detection, ODR configuration, and range
settings lays the groundwork for the activity/inactivity hysteresis
mechanism, which will be implemented in a subsequent patch. As such,
portions of this patch prepare switch-case structures to support those
upcoming changes.

Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Link: https://patch.msgid.link/20250727210014.27766-3-l.rubusch@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>