iio: imu: st_lsm6dsx: add support to LSM6DSTX

Add support to STM LSM6DSTX (acc + gyro) Mems sensor.
The LSM6DSTX sensor can use LSM6DST as fallback device since it
implements all the LSM6DSTX features currently implemented in
st_lsm6dsx.

Datasheet: https://www.st.com/resource/en/datasheet/lsm6dstx.pdf


Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/573a232b0f320b2ec92c56fa24393cfb275183fb.1663262890.git.lorenzo@kernel.org


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