iio: imu: st_lsm6dsx: add support to LSM6DST

Add support to STM LSM6DST (acc + gyro) Mems sensor
https://www.st.com/resource/en/datasheet/lsm6dst.pdf



Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/835127b76ef5ad05aa2aac58298aee5f3073fb71.1601985763.git.lorenzo@kernel.org


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