Documentation: bindings: document the Berlin ADC driver

Following the addition of a Berlin ADC driver, this patch adds the
corresponding bindings documentation.
Signed-off-by: NAntoine Tenart <antoine.tenart@free-electrons.com>
Acked-by: NJonathan Cameron <jic23@kernel.org>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/adc/berlin2_adc.txt

+* Berlin Analog to Digital Converter (ADC)
+
+The Berlin ADC has 8 channels, with one connected to a temperature sensor.
+It is part of the system controller register set. The ADC node should be a
+sub-node of the system controller node.
+
+Required properties:
+- compatible: must be "marvell,berlin2-adc"
+- interrupts: the interrupts for the ADC and the temperature sensor
+- interrupt-names: should be "adc" and "tsen"
+
+Example:
+
+adc: adc {
+	compatible = "marvell,berlin2-adc";
+	interrupt-parent = <&sic>;
+	interrupts = <12>, <14>;
+	interrupt-names = "adc", "tsen";
+};