dt-bindings: mips: lantiq: Document Lantiq Xway PMU bindings

Document the Lantiq Xway SoC series Power Management Unit (PMU) bindings.
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

Documentation/devicetree/bindings/mips/lantiq/lantiq,pmu.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mips/lantiq/lantiq,pmu.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Lantiq Xway SoC series Power Management Unit (PMU)
+
+maintainers:
+  - John Crispin <john@phrozen.org>
+
+properties:
+  compatible:
+    items:
+      - enum:
+          - lantiq,pmu-xway
+
+  reg:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    pmu@102000 {
+        compatible = "lantiq,pmu-xway";
+        reg = <0x102000 0x1000>;
+    };