dt-bindings: iio: st,lsm6dsx: reference common schema for mount-matrix

Reference iio.yaml schema from dtschema to allow already used
mount-matrix property:

  msm8953-xiaomi-daisy.dtb: imu@6a: Unevaluated properties are not allowed ('mount-matrix' was unexpected)
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230507173923.263741-1-krzysztof.kozlowski@linaro.orgSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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

@@ -98,6 +98,7 @@ required:
   - reg
 
 allOf:
+  - $ref: /schemas/iio/iio.yaml#
   - $ref: /schemas/spi/spi-peripheral-props.yaml#
 
 unevaluatedProperties: false