dt-bindings: mfd: Add binding for qnap,ts433-mcu devices

These MCUs can be found in network attached storage devices made by QNAP.
They are connected to a serial port of the host device and provide
functionality like LEDs, power-control and temperature monitoring.

LEDs, buttons, etc are all elements of the MCU firmware itself, so don't
need devicetree input, though the fan gets its cooling settings from
a fan-0 subnode.

A binding for the LEDs for setting the linux-default-trigger may come
later, once all the LEDs are understood and ATA controllers actually
can address individual port-LEDs, but are really optional.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20241107114712.538976-4-heiko@sntech.de
Signed-off-by: Lee Jones <lee@kernel.org>

Documentation/devicetree/bindings/mfd/qnap,ts433-mcu.yaml

@@ -0,0 +1,42 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mfd/qnap,ts433-mcu.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: QNAP NAS on-board Microcontroller
+
+maintainers:
+  - Heiko Stuebner <heiko@sntech.de>
+
+description:
+  QNAP embeds a microcontroller on their NAS devices adding system feature
+  as PWM Fan control, additional LEDs, power button status and more.
+
+properties:
+  compatible:
+    enum:
+      - qnap,ts433-mcu
+
+patternProperties:
+  "^fan-[0-9]+$":
+    $ref: /schemas/hwmon/fan-common.yaml#
+    unevaluatedProperties: false
+
+required:
+  - compatible
+
+additionalProperties: false
+
+examples:
+  - |
+    uart {
+      mcu {
+        compatible = "qnap,ts433-mcu";
+
+        fan-0 {
+          #cooling-cells = <2>;
+          cooling-levels = <0 64 89 128 166 204 221 238>;
+        };
+      };
+    };