dt-bindings: adi,ad5791: Add support for controlling RBUF

Added new property to support an external amplifier to be connected in a
gain of two configuration.
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20231129-ad5791-michael-stuff-v3-1-48e192b00909@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/dac/adi,ad5791.yaml

@@ -26,6 +26,11 @@ properties:
   vdd-supply: true
   vss-supply: true
 
+  adi,rbuf-gain2-en:
+    description: Specify to allow an external amplifier to be connected in a
+      gain of two configuration.
+    type: boolean
+
 required:
   - compatible
   - reg