iio: imu: bmi270: add support for motion events

iio: imu: bmi270: add support for motion events

Any-motion event can be enabled on a per-axis basis and triggers a
combined event when motion is detected on any axis.

No-motion event is triggered if the rate of change on all axes falls
below a specified threshold for a configurable duration. A fake channel
is used to report this event.

Threshold and duration can be configured from userspace.

Signed-off-by: Gustavo Silva <gustavograzs@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>