iio: imu: st_lsm6dsx: move iio chan definitions in st_lsm6dsx_sensor_settings

Move IIO channel definitions in st_lsm6dsx_sensor_settings in order to
support sensors with different channels maps.
This is a preliminary patch to add support for LSM9DS1 sensor to
st_lsm6dsx driver.

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