iio: accel: adxl313: add inactivity sensing

iio: accel: adxl313: add inactivity sensing

Enhance the interrupt handler to process inactivity events. Introduce
functions to configure the threshold and period registers for
inactivity detection, as well as to enable or disable the inactivity
feature. Extend the fake IIO channel to handle inactivity events by
combining the x, y, and z axes using a logical AND operation.

Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20250702230819.19353-6-l.rubusch@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>