dt-bindings: arm/npcm: Add binding for global control registers (GCR)

A nuvoton,*-gcr node is present in nuvoton-common-npcm7xx.dtsi and will
be added to nuvoton-wpcm450.dtsi. It is necessary for the NPCM7xx and
WPCM450 pinctrl drivers, and may later be used to retrieve SoC model and
version information.

This patch adds a binding to describe this node.
Signed-off-by: NJonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: NRob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220129115228.2257310-2-j.neuschaefer@gmx.netSigned-off-by: NJoel Stanley <joel@jms.id.au>

Documentation/devicetree/bindings/arm/npcm/nuvoton,gcr.yaml

+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/arm/npcm/nuvoton,gcr.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Global Control Registers block in Nuvoton SoCs
+
+maintainers:
+  - Jonathan Neuschäfer <j.neuschaefer@gmx.net>
+
+description:
+  The Global Control Registers (GCR) are a block of registers in Nuvoton SoCs
+  that expose misc functionality such as chip model and version information or
+  pinmux settings.
+
+properties:
+  compatible:
+    items:
+      - enum:
+          - nuvoton,wpcm450-gcr
+          - nuvoton,npcm750-gcr
+      - const: syscon
+      - const: simple-mfd
+
+  reg:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+
+additionalProperties:
+  type: object
+
+examples:
+  - |
+    gcr: syscon@800000 {
+      compatible = "nuvoton,npcm750-gcr", "syscon", "simple-mfd";
+      reg = <0x800000 0x1000>;
+
+      mux-controller {
+        compatible = "mmio-mux";
+        #mux-control-cells = <1>;
+        mux-reg-masks = <0x38 0x07>;
+        idle-states = <2>;
+      };
+    };