iio: accel: adxl345: add coupling detection for activity/inactivity

iio: accel: adxl345: add coupling detection for activity/inactivity

Enable AC/DC coupling configuration for activity and inactivity detection
by setting the AC/DC bit. Extend existing magnitude-based detection with
adaptive AC-coupled mode.

Use DC-coupled mode to compare acceleration samples directly against
configured thresholds. Use AC-coupled mode to compare samples against a
reference taken at the start of activity detection. Implement DC-coupled
events using MAG, and AC-coupled events using MAG_ADAPTIVE.

Expose configuration of thresholds and periods via separate sysfs handles.
Note that both coupling modes share the same sensor registers, so activity
or inactivity detection cannot be configured for both AC and DC
simultaneously. Apply the most recently configured mode.

Simplify event handling and support adaptive AC-coupling.

Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Link: https://patch.msgid.link/20250727210014.27766-5-l.rubusch@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>