thermal: generic-adc: Add DT binding for ADC based thermal sensor

Sometimes, thermal sensors like NCT thermistors are connected to
the ADC channel. The temperature is read by reading the voltage
across the sensor resistance via ADC and referring the lookup
table for ADC value to temperature.

Add DT binding doc for the ADC based thermal sensor driver to
detail the DT property and provide the example for how to use it.
Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Rob Herring <rob@kernel.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>

Documentation/devicetree/bindings/thermal/thermal-generic-adc.txt

+General Purpose Analog To Digital Converter (ADC) based thermal sensor.
+
+On some of platforms, thermal sensor like thermistors are connected to
+one of ADC channel and sensor resistance is read via voltage across the
+sensor resistor. The voltage read across the sensor is mapped to
+temperature using voltage-temperature lookup table.
+
+Required properties:
+===================
+- compatible:		     Must be "generic-adc-thermal".
+- temperature-lookup-table:  Two dimensional array of Integer; lookup table
+			     to map the relation between ADC value and
+			     temperature. When ADC is read, the value is
+			     looked up on the table to get the equivalent
+			     temperature.
+			     The first value of the each row of array is the
+			     temperature in milliCelsius and second value of
+			     the each row of array is the ADC read value.
+- #thermal-sensor-cells:     Should be 1. See ./thermal.txt for a description
+			     of this property.
+
+Example :
+#include <dt-bindings/thermal/thermal.h>
+
+i2c@7000c400 {
+	ads1015: ads1015@4a {
+		reg = <0x4a>;
+		compatible = "ads1015";
+		sampling-frequency = <3300>;
+		#io-channel-cells = <1>;
+	};
+};
+
+tboard_thermistor: thermal-sensor {
+	compatible = "generic-adc-thermal";
+	#thermal-sensor-cells = <0>;
+	io-channels = <&ads1015 1>;
+	io-channel-names = "sensor-channel";
+	temperature-lookup-table = <    (-40000) 2578
+					(-39000) 2577
+					(-38000) 2576
+					(-37000) 2575
+					(-36000) 2574
+					(-35000) 2573
+					(-34000) 2572
+					(-33000) 2571
+					(-32000) 2569
+					(-31000) 2568
+					(-30000) 2567
+					::::::::::
+					118000 254
+					119000 247
+					120000 240
+					121000 233
+					122000 226
+					123000 220
+					124000 214
+					125000 208>;
+};
+
+dummy_cool_dev: dummy-cool-dev {
+	compatible = "dummy-cooling-dev";
+	#cooling-cells = <2>; /* min followed by max */
+};
+
+thermal-zones {
+	Tboard {
+		polling-delay = <15000>; /* milliseconds */
+		polling-delay-passive = <0>; /* milliseconds */
+		thermal-sensors = <&tboard_thermistor>;
+
+		trips {
+			therm_est_trip: therm_est_trip {
+				temperature = <40000>;
+				type = "active";
+				hysteresis = <1000>;
+			};
+		};
+
+		cooling-maps {
+			map0 {
+				trip = <&therm_est_trip>;
+				cooling-device = <&dummy_cool_dev THERMAL_NO_LIMIT THERMAL_NO_LIMIT>;
+				contribution = <100>;
+			};
+
+		};
+	};
+};