dt-bindings: soc: ti: am654-serdes-ctrl: Add simple-mfd to compatible items

This node contains a child which is only probed if simple-mfd is in the
compatible list. Add this here.

Signed-off-by: Andrew Davis <[e-mail]>
Acked-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,am654-serdes-ctrl.yaml

···
     items:
       - const: ti,am654-serdes-ctrl
       - const: syscon
+      - const: simple-mfd
 
   reg:
     maxItems: 1
···
 examples:
   - |
     clock@4080 {
-        compatible = "ti,am654-serdes-ctrl", "syscon";
+        compatible = "ti,am654-serdes-ctrl", "syscon", "simple-mfd";
         reg = <0x4080 0x4>;
 
         mux-controller {