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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

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

···
       - items:
           - const: st,asm330lhhx
           - const: st,lsm6dsr
+      - items:
+          - const: st,lsm6dstx
+          - const: st,lsm6dst
 
   reg:
     maxItems: 1