dt-bindings: pwm: argon40,fan-hat: Document Argon40 Fan HAT

Document trivial PWM on Argon40 Fan HAT, which is a RaspberryPi
blower fan hat which can be controlled over I2C.

Reviewed-by: Rob Herring (Arm) <[e-mail]>
Signed-off-by: Marek Vasut <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Uwe Kleine-König <[e-mail]>

Documentation/devicetree/bindings/pwm/argon40,fan-hat.yaml

···
+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/pwm/argon40,fan-hat.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Argon40 Fan HAT PWM controller
+
+maintainers:
+  - Marek Vasut <[e-mail]>
+
+description:
+  The trivial PWM on Argon40 Fan HAT, which is a RaspberryPi blower fan
+  hat which can be controlled over I2C, generates a fixed 30 kHz period
+  PWM signal with configurable 0..100% duty cycle to control the fan
+  speed.
+
+allOf:
+  - $ref: pwm.yaml#
+
+properties:
+  compatible:
+    const: argon40,fan-hat
+
+  reg:
+    maxItems: 1
+
+  "#pwm-cells":
+    const: 3
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    i2c {
+      #address-cells = <1>;
+      #size-cells = <0>;
+
+      pwm@1a {
+        compatible = "argon40,fan-hat";
+        reg = <0x1a>;
+        #pwm-cells = <3>;
+      };
+    };