dt-bindings: iio: stm32-adc: add option to set resolution

Add documentation for 'assigned-resolution-bits' dt optional property.
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/adc/st,stm32-adc.txt

@@ -57,6 +57,9 @@ Optional properties:
 - dmas: Phandle to dma channel for this ADC instance.
   See ../../dma/dma.txt for details.
 - dma-names: Must be "rx" when dmas property is being used.
+- assigned-resolution-bits: Resolution (bits) to use for conversions. Must
+  match device available resolutions (e.g. can be 6, 8, 10 or 12 on stm32f4).
+  Default is maximum resolution if unset.
 
 Example:
 	adc: adc@40012000 {
@@ -84,6 +87,7 @@ Example:
 			st,adc-channels = <8>;
 			dmas = <&dma2 0 0 0x400 0x0>;
 			dma-names = "rx";
+			assigned-resolution-bits = <8>;
 		};
 		...
 		other adc child nodes follow...