dt-bindings: thermal: Add yaml bindings for thermal cooling-devices

As part of moving the thermal bindings to YAML, split it up into 3
bindings: thermal sensors, cooling devices and thermal zones.

The property #cooling-cells is required in each device that acts as a
cooling device - whether active or passive. So any device that can
throttle its performance to passively reduce heat dissipation (e.g.
CPUs, GPUs) and any device that can actively dissipate heat at different
levels (e.g. fans) will contain this property.
Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/7a9ead7fb67585fb70ab3ffd481e7d567e96970e.1585748882.git.amit.kucheria@linaro.org

Documentation/devicetree/bindings/thermal/thermal-cooling-devices.yaml

+# SPDX-License-Identifier: (GPL-2.0)
+# Copyright 2020 Linaro Ltd.
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/thermal/thermal-cooling-devices.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Thermal cooling device binding
+
+maintainers:
+  - Amit Kucheria <amitk@kernel.org>
+
+description: |
+  Thermal management is achieved in devicetree by describing the sensor hardware
+  and the software abstraction of cooling devices and thermal zones required to
+  take appropriate action to mitigate thermal overload.
+
+  The following node types are used to completely describe a thermal management
+  system in devicetree:
+   - thermal-sensor: device that measures temperature, has SoC-specific bindings
+   - cooling-device: device used to dissipate heat either passively or actively
+   - thermal-zones: a container of the following node types used to describe all
+     thermal data for the platform
+
+  This binding describes the cooling devices.
+
+  There are essentially two ways to provide control on power dissipation:
+    - Passive cooling: by means of regulating device performance. A typical
+      passive cooling mechanism is a CPU that has dynamic voltage and frequency
+      scaling (DVFS), and uses lower frequencies as cooling states.
+    - Active cooling: by means of activating devices in order to remove the
+      dissipated heat, e.g. regulating fan speeds.
+
+  Any cooling device has a range of cooling states (i.e. different levels of
+  heat dissipation). They also have a way to determine the state of cooling in
+  which the device is. For example, a fan's cooling states correspond to the
+  different fan speeds possible. Cooling states are referred to by single
+  unsigned integers, where larger numbers mean greater heat dissipation. The
+  precise set of cooling states associated with a device should be defined in
+  a particular device's binding.
+
+select: true
+
+properties:
+  "#cooling-cells":
+    description:
+        Must be 2, in order to specify minimum and maximum cooling state used in
+        the cooling-maps reference. The first cell is the minimum cooling state
+        and the second cell is the maximum cooling state requested.
+    const: 2
+
+examples:
+  - |
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+    #include <dt-bindings/thermal/thermal.h>
+
+    // Example 1: Cpufreq cooling device on CPU0
+    cpus {
+            #address-cells = <2>;
+            #size-cells = <0>;
+
+            CPU0: cpu@0 {
+                    device_type = "cpu";
+                    compatible = "qcom,kryo385";
+                    reg = <0x0 0x0>;
+                    enable-method = "psci";
+                    cpu-idle-states = <&LITTLE_CPU_SLEEP_0
+                                       &LITTLE_CPU_SLEEP_1
+                                       &CLUSTER_SLEEP_0>;
+                    capacity-dmips-mhz = <607>;
+                    dynamic-power-coefficient = <100>;
+                    qcom,freq-domain = <&cpufreq_hw 0>;
+                    #cooling-cells = <2>;
+                    next-level-cache = <&L2_0>;
+                    L2_0: l2-cache {
+                            compatible = "cache";
+                            next-level-cache = <&L3_0>;
+                            L3_0: l3-cache {
+                                    compatible = "cache";
+                            };
+                    };
+          };
+
+          /* ... */
+
+    };
+
+    /* ... */
+
+    thermal-zones {
+            cpu0-thermal {
+                    polling-delay-passive = <250>;
+                    polling-delay = <1000>;
+
+                    thermal-sensors = <&tsens0 1>;
+
+                    trips {
+                            cpu0_alert0: trip-point0 {
+                                    temperature = <90000>;
+                                    hysteresis = <2000>;
+                                    type = "passive";
+                            };
+                    };
+
+                    cooling-maps {
+                            map0 {
+                                    trip = <&cpu0_alert0>;
+                                    /* Corresponds to 1000MHz in OPP table */
+                                    cooling-device = <&CPU0 5 5>;
+                            };
+                    };
+            };
+
+            /* ... */
+    };
+...