dt-bindings: iio/adc: Add a compatible string for JZ4770 SoC ADC

Add a compatible string for the ADC controller present on
Ingenic JZ4770 SoC.
Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/ingenic,adc.txt

@@ -5,6 +5,7 @@ Required properties:
 - compatible: Should be one of:
   * ingenic,jz4725b-adc
   * ingenic,jz4740-adc
+  * ingenic,jz4770-adc
 - reg: ADC controller registers location and length.
 - clocks: phandle to the SoC's ADC clock.
 - clock-names: Must be set to "adc".