dt-bindings: iio: adc: Add max9611 ADC

Add device tree bindings documentation for Maxim max9611/max9612 current
sense amplifier.
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/adc/max9611.txt

+* Maxim max9611/max9612 current sense amplifier with 12-bits ADC interface
+
+Maxim max9611/max9612 is an high-side current sense amplifier with integrated
+12-bits ADC communicating over I2c bus.
+The device node for this driver shall be a child of a I2c controller.
+
+Required properties
+  - compatible: Should be "maxim,max9611" or "maxim,max9612"
+  - reg: The 7-bits long I2c address of the device
+  - shunt-resistor-micro-ohms: Value, in micro Ohms, of the current sense shunt
+			        resistor
+
+Example:
+
+&i2c4 {
+	csa: adc@7c {
+		compatible = "maxim,max9611";
+		reg = <0x7c>;
+
+		shunt-resistor-micro-ohms = <5000>;
+	};
+};
+
+This device node describes a current sense amplifier sitting on I2c4 bus
+with address 0x7c (read address is 0xf9, write address is 0xf8).
+A sense resistor of 0,005 Ohm is installed between RS+ and RS- current-sensing
+inputs.