iio: accel: adxl345: extend sample frequency adjustments

iio: accel: adxl345: extend sample frequency adjustments

Introduce enums and functions to work with the sample frequency
adjustments. Let the sample frequency adjust via IIO and configure
a reasonable default.

Replace the old static sample frequency handling. During adjustment of
bw registers, measuring is disabled and afterwards enabled again.

Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Link: https://patch.msgid.link/20250510224405.17910-2-l.rubusch@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>