Documentation: bindings: document the Berlin ADC driver

* 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";

};