dt-bindings: iio: adc: Add AD7616 ADC documentation

Document support for AD7616 Analog to Digital Converter.
Signed-off-by: Beniamin Bia <beniamin.bia@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/adi,ad7606.txt

@@ -7,6 +7,7 @@ Required properties for the AD7606:
 	* "adi,ad7606-8"
 	* "adi,ad7606-6"
 	* "adi,ad7606-4"
+	* "adi,ad7616"
 - reg: SPI chip select number for the device
 - spi-max-frequency: Max SPI frequency to use
 	see: Documentation/devicetree/bindings/spi/spi-bus.txt