dt-bindings: arm: Add missing APM X-Gene SoC platforms

APM X-Gene SoCs have been in use for some time, but the root compatibles
have not been documented. Add them.

Link: https://patch.msgid.link/20251022181127.4147542-1-robh@kernel.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

Documentation/devicetree/bindings/arm/apm.yaml

@@ -0,0 +1,28 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/arm/apm.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: APM X-Gene SoC Platforms
+
+maintainers:
+  - Khuong Dinh <khuong@os.amperecomputing.com>
+
+properties:
+  $nodename:
+    const: "/"
+  compatible:
+    oneOf:
+      - description: Boards with X-Gene1 Soc
+        items:
+          - const: apm,mustang
+          - const: apm,xgene-storm
+
+      - description: Boards with X-Gene2 SoC
+        items:
+          - const: apm,merlin
+          - const: apm,xgene-shadowcat
+
+additionalProperties: true
+...