dt-bindings: mfd: Add Realtek RTD1xxx system controllers

The Realtek system controllers are already in use with only generic
'"syscon", "simple-mfd"' compatibles and are missing specific
compatibles as required. Add a schema with specific compatibles. The
labels used in .dts files serve as the basis for the names.

It's doubtful the child nodes upstream are complete and I don't have
documentation, so the specific child nodes aren't documented here. Of
the ones in use, bindings already exist for them.

Link: https://patch.msgid.link/20251215212624.3319681-1-robh@kernel.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

Documentation/devicetree/bindings/mfd/realtek,rtd1xxx.yaml

@@ -0,0 +1,69 @@
+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mfd/realtek,rtd1xxx.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Realtek RTD1xxx system controllers
+
+maintainers:
+  - Andreas Färber <afaerber@suse.de>
+
+properties:
+  compatible:
+    items:
+      - enum:
+          - realtek,rtd1293-crt
+          - realtek,rtd1293-iso
+          - realtek,rtd1293-misc
+          - realtek,rtd1293-sb2
+          - realtek,rtd1293-scpu-wrapper
+          - realtek,rtd1295-crt
+          - realtek,rtd1295-iso
+          - realtek,rtd1295-misc
+          - realtek,rtd1295-sb2
+          - realtek,rtd1295-scpu-wrapper
+          - realtek,rtd1296-crt
+          - realtek,rtd1296-iso
+          - realtek,rtd1296-misc
+          - realtek,rtd1296-sb2
+          - realtek,rtd1296-scpu-wrapper
+          - realtek,rtd1395-crt
+          - realtek,rtd1395-iso
+          - realtek,rtd1395-misc
+          - realtek,rtd1395-sb2
+          - realtek,rtd1395-scpu-wrapper
+          - realtek,rtd1619-crt
+          - realtek,rtd1619-iso
+          - realtek,rtd1619-misc
+          - realtek,rtd1619-sb2
+          - realtek,rtd1619-scpu-wrapper
+      - const: syscon
+      - const: simple-mfd
+
+  reg:
+    maxItems: 1
+
+  reg-io-width:
+    const: 4
+
+  ranges: true
+
+  '#address-cells':
+    const: 1
+
+  '#size-cells':
+    const: 1
+
+patternProperties:
+  '@[0-9a-f]+$':
+    type: object
+
+    required:
+      - compatible
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false