dt-binding: iio: Add optional label property

This optional property defines a symbolic name for the device.
This helps to distinguish between more than one iio device
of the same type.
Suggested-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Phil Reid <preid@electromag.com.au>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/iio-bindings.txt

@@ -18,12 +18,17 @@ Required properties:
 		   with a single IIO output and 1 for nodes with multiple
 		   IIO outputs.
 
+Optional properties:
+label:		   A symbolic name for the device.
+
+
 Example for a simple configuration with no trigger:
 
 	adc: voltage-sensor@35 {
 		compatible = "maxim,max1139";
 		reg = <0x35>;
 		#io-channel-cells = <1>;
+		label = "voltage_feedback_group1";
 	};
 
 Example for a configuration with trigger: