iio: imu: st_lsm6dsx: add support to multiple devices with the same settings

Add capability to support multiple devices with the same
st_lsm6dsx_settings like LSM6DSM/LSM6DSL

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>