dt-bindings: iio: accel: kxcjk1013: Document mount-matrix property

The generic IIO mount-matrix property conveys physical orientation of the
hardware chip.
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/accel/kionix,kxcjk1013.txt

@@ -9,9 +9,16 @@ Required properties:
     "kionix,kxtf9"
  - reg: i2c slave address
 
+Optional properties:
+
+  - mount-matrix: an optional 3x3 mounting rotation matrix
+
 Example:
 
 kxtf9@f {
 	compatible = "kionix,kxtf9";
 	reg = <0x0F>;
+	mount-matrix =	"0", "1", "0",
+			"1", "0", "0",
+			"0", "0", "1";
 };