iio: accel: kxsd9: Add device tree bindings

This accelerometer can be probed from the device tree, so it needs
to have proper documentation of it's device tree bindings.

Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/accel/kionix,kxsd9.txt

+Kionix KXSD9 Accelerometer device tree bindings
+
+Required properties:
+ - compatible: 		should be set to "kionix,kxsd9"
+ - reg:			i2c slave address
+
+Optional properties:
+ - vdd-supply:		The input supply for VDD
+ - iovdd-supply:	The input supply for IOVDD
+ - interrupts:		The movement detection interrupt
+ - mount-matrix:	See mount-matrix.txt
+
+Example:
+
+kxsd9@18 {
+	compatible = "kionix,kxsd9";
+	reg = <0x18>;
+	interrupt-parent = <&foo>;
+	interrupts = <57 IRQ_TYPE_EDGE_FALLING>;
+	iovdd-supply = <&bar>;
+	vdd-supply = <&baz>;
+};