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

Documentation/devicetree/bindings/iio/imu/st,lsm6dsx.yaml

@@ -35,6 +35,9 @@ properties:
       - items:
           - const: st,asm330lhhx
           - const: st,lsm6dsr
+      - items:
+          - const: st,lsm6dstx
+          - const: st,lsm6dst
 
   reg:
     maxItems: 1