dt-bindings: bus: aspeed: Require syscon for AST2600 AHB controller

The AST2600's ACRY (eliptic curve and RSA crypto engine) requires access
to configuration exposed by the AHB controller. The devicetree already
describes the AHB controller node as a syscon, so require this in the
binding to satisfy the ACRY relationship.

Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Link: https://patch.msgid.link/20251211-dev-dt-warnings-all-v1-9-21b18b9ada77@codeconstruct.com.au
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

Documentation/devicetree/bindings/bus/aspeed,ast2600-ahbc.yaml

@@ -17,8 +17,10 @@ description: |
 
 properties:
   compatible:
-    enum:
-      - aspeed,ast2600-ahbc
+    items:
+      - enum:
+          - aspeed,ast2600-ahbc
+      - const: syscon
 
   reg:
     maxItems: 1
@@ -32,6 +34,6 @@ additionalProperties: false
 examples:
   - |
     ahbc@1e600000 {
-        compatible = "aspeed,ast2600-ahbc";
+        compatible = "aspeed,ast2600-ahbc", "syscon";
         reg = <0x1e600000 0x100>;
     };