dt-bindings: mfd: Add Khadas Microcontroller bindings

This Microcontroller is present on the Khadas VIM1, VIM2, VIM3 and Edge
boards.

It has multiple boot control features like password check, power-on
options, power-off control and system FAN control on recent boards.
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/khadas,mcu.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mfd/khadas,mcu.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Khadas on-board Microcontroller Device Tree Bindings
+
+maintainers:
+  - Neil Armstrong <narmstrong@baylibre.com>
+
+description: |
+  Khadas embeds a microcontroller on their VIM and Edge boards adding some
+  system feature as PWM Fan control (for VIM2 rev14 or VIM3), User memory
+  storage, IR/Key resume control, system power LED control and more.
+
+properties:
+  compatible:
+    enum:
+      - khadas,mcu # MCU revision is discoverable
+
+  "#cooling-cells": # Only needed for boards having FAN control feature
+    const: 2
+
+  reg:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    i2c {
+      #address-cells = <1>;
+      #size-cells = <0>;
+      khadas_mcu: system-controller@18 {
+        compatible = "khadas,mcu";
+        reg = <0x18>;
+        #cooling-cells = <2>;
+      };
+    };