iio: imu: st_lsm6dsx: move register definitions to sensor_settings struct

Move some register definitions to the per-device array of struct
st_lsm6dsx_sensor_settings in order to simplify adding new sensor
devices to the driver.

Also, remove completely unused register definitions.

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>