dt-bindings: mfd: Document the Xylon LogiCVC multi-function device

The LogiCVC is a display engine which also exposes GPIO functionality.
For this reason, it is described as a multi-function device that is expected
to provide register access to its children nodes for gpio and display.
Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191203141243.251058-3-paul.kocialkowski@bootlin.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/mfd/xylon,logicvc.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+# Copyright 2019 Bootlin
+%YAML 1.2
+---
+$id: "http://devicetree.org/schemas/mfd/xylon,logicvc.yaml#"
+$schema: "http://devicetree.org/meta-schemas/core.yaml#"
+
+title: Xylon LogiCVC multi-function device
+
+maintainers:
+  - Paul Kocialkowski <paul.kocialkowski@bootlin.com>
+
+description: |
+  The LogiCVC is a display controller that also contains a GPIO controller.
+  As a result, a multi-function device is exposed as parent of the display
+  and GPIO blocks.
+
+properties:
+  compatible:
+    items:
+      - enum:
+          - xylon,logicvc-3.02.a
+      - const: syscon
+      - const: simple-mfd
+
+  reg:
+    maxItems: 1
+
+select:
+  properties:
+    compatible:
+      contains:
+        enum:
+          - xylon,logicvc-3.02.a
+
+  required:
+    - compatible
+
+required:
+  - compatible
+  - reg
+
+examples:
+  - |
+    logicvc: logicvc@43c00000 {
+      compatible = "xylon,logicvc-3.02.a", "syscon", "simple-mfd";
+      reg = <0x43c00000 0x6000>;
+      #address-cells = <1>;
+      #size-cells = <1>;
+    };