dt-bindings: mfd: ti,j721e-system-controller: Add compatible string for AM654

Add the child nodes that can be found under this node. Then as done
for other similar devices (J7200 and J721s2) add support for the AM654
system controller to this binding.

Signed-off-by: Andrew Davis <[e-mail]>
Reviewed-by: Rob Herring (Arm) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Nishanth Menon <[e-mail]>

Documentation/devicetree/bindings/soc/ti/ti,j721e-system-controller.yaml

···
   compatible:
     items:
       - enum:
+          - ti,am654-system-controller
           - ti,j7200-system-controller
           - ti,j721e-system-controller
           - ti,j721s2-system-controller
···
     type: object
     description:
       The node corresponding to PCIe control register.
+
+  "^clock@[0-9a-f]+$":
+    type: object
+    $ref: /schemas/soc/ti/ti,am654-serdes-ctrl.yaml#
+    description:
+      This is the Serdes Control region.
+
+  "^dss-oldi-io-ctrl@[0-9a-f]+$":
+    type: object
+    $ref: /schemas/mfd/syscon.yaml#
+    description:
+      This is the DSS OLDI CTRL region.
 
 required:
   - compatible