dt-bindings: iio: imu: st_lsm6dsx: add lsm6dstx device bindings

Add device bindings for lsm6dstx IMU sensor.
Use lsm6dst as fallback device for lsm6dstx since it implements all the
features currently supported by lsm6dstx.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/dadc2acf88b0b73feac9a8980ad9af1658297bc1.1663262890.git.lorenzo@kernel.org


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