dt-bindings: memory-controllers: fsl,imx8m-ddrc: restrict opp-table to objects

Simple 'opp-table:true' accepts a boolean property as opp-table, so
restrict it to object to properly enferce real OPP table nodes.
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220818061549.9087-1-krzysztof.kozlowski@linaro.org

Documentation/devicetree/bindings/memory-controllers/fsl/imx8m-ddrc.yaml

@@ -47,7 +47,8 @@ properties:
       - const: apb
 
   operating-points-v2: true
-  opp-table: true
+  opp-table:
+    type: object
 
 required:
   - reg