iio: imu: bmi270: Add scale and sampling frequency to BMI270 IMU

Add read and write functions and create _available entries.

Signed-off-by: Justin Weiss <justin@justinweiss.com>
Link: https://patch.msgid.link/20241027172029.160134-3-justin@justinweiss.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>