iio: accel: adxl313: add activity sensing

iio: accel: adxl313: add activity sensing

Add support for configuring an activity detection threshold. Extend the
interrupt handler to process activity-related interrupts, and provide
functions to set the threshold as well as to enable or disable activity
sensing. Additionally, introduce a virtual channel that represents the
logical AND of the x, y, and z axes in the IIO channel.

This patch serves as a preparatory step; some definitions and functions
introduced here are intended to be extended later to support inactivity
detection.

Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20250702230819.19353-5-l.rubusch@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>