dt-bindings: iio: afe: add bindings for temperature-sense-rtd

An ADC is often used to measure other quantities indirectly. This
binding describe one case, the measurement of a temperature through the
voltage across an RTD resistor such as a PT1000.
Signed-off-by: Liam Beguin <liambeguin@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220213025739.2561834-10-liambeguin@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/afe/temperature-sense-rtd.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/afe/temperature-sense-rtd.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Temperature Sense RTD
+
+maintainers:
+  - Liam Beguin <liambeguin@gmail.com>
+
+description: |
+  RTDs (Resistance Temperature Detectors) are a kind of temperature sensors
+  used to get a linear voltage to temperature reading within a give range
+  (usually 0 to 100 degrees Celsius).
+
+  When an io-channel measures the output voltage across an RTD such as a
+  PT1000, the interesting measurement is almost always the corresponding
+  temperature, not the voltage output. This binding describes such a circuit.
+
+  The general transfer function here is (using SI units)
+
+    V = R(T) * iexc
+    R(T) = r0 * (1 + alpha * T)
+    T = 1 / (alpha * r0 * iexc) * (V - r0 * iexc)
+
+  The following circuit matches what's in the examples section.
+
+           5V0
+          -----
+            |
+        +---+----+
+        |  R 5k  |
+        +---+----+
+            |
+            V 1mA
+            |
+            +---- Vout
+            |
+        +---+----+
+        | PT1000 |
+        +---+----+
+            |
+          -----
+           GND
+
+properties:
+  compatible:
+    const: temperature-sense-rtd
+
+  io-channels:
+    maxItems: 1
+    description: |
+      Channel node of a voltage io-channel.
+
+  '#io-channel-cells':
+    const: 0
+
+  excitation-current-microamp:
+    description: The current fed through the RTD sensor.
+
+  alpha-ppm-per-celsius:
+    description: |
+      alpha can also be expressed in micro-ohms per ohm Celsius. It's a linear
+      approximation of the resistance versus temperature relationship
+      between 0 and 100 degrees Celsius.
+
+      alpha = (R_100 - R_0) / (100 * R_0)
+
+      Where, R_100 is the resistance of the sensor at 100 degrees Celsius, and
+      R_0 (or r-naught-ohms) is the resistance of the sensor at 0 degrees
+      Celsius.
+
+      Pure platinum has an alpha of 3925. Industry standards such as IEC60751
+      and ASTM E-1137 specify an alpha of 3850.
+
+  r-naught-ohms:
+    description: |
+      Resistance of the sensor at 0 degrees Celsius.
+      Common values are 100 for PT100, 500 for PT500, and 1000 for PT1000
+
+additionalProperties: false
+required:
+  - compatible
+  - io-channels
+  - excitation-current-microamp
+  - alpha-ppm-per-celsius
+  - r-naught-ohms
+
+examples:
+  - |
+    pt1000_1: temperature-sensor0 {
+        compatible = "temperature-sense-rtd";
+        #io-channel-cells = <0>;
+        io-channels = <&temp_adc1 0>;
+
+        excitation-current-microamp = <1000>; /* i = U/R = 5 / 5000 */
+        alpha-ppm-per-celsius = <3908>;
+        r-naught-ohms = <1000>;
+    };
+...