iio: accel: adxl345: add inactivity feature

Add support for the sensor’s inactivity feature in the driver. When both
activity and inactivity detection are enabled, the sensor sets a link bit
that ties the two functions together. This also enables auto-sleep mode,
allowing the sensor to automatically enter sleep state upon detecting
inactivity.

Inactivity detection relies on a configurable threshold and a specified
time period. If sensor measurements remain below the threshold for the
defined duration, the sensor transitions to the inactivity state.

When an Output Data Rate (ODR) is set, the inactivity time period is
automatically adjusted to a sensible default. Higher ODRs result in shorter
inactivity timeouts, while lower ODRs allow longer durations-within
reasonable upper and lower bounds. This is important because features like
auto-sleep operate effectively only between 12.5 Hz and 400 Hz. These
defaults are applied when the sample rate is modified, but users can
override them by explicitly setting a custom inactivity timeout.

Similarly, configuring the g-range provides default threshold values for
both activity and inactivity detection. These are implicit defaults meant
to simplify configuration, but they can also be manually overridden as
needed.

Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Link: https://patch.msgid.link/20250727210014.27766-4-l.rubusch@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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