dt-bindings: cpufreq: apple,cluster-cpufreq: Add t6020 compatible

The CPU frequency control on M2 Pro/Max/Ultra SoCs is compatible to
the M2 (T8112) one. So add this with the per-SoC compatible to the
bindings.

Reviewed-by: Neal Gompa <[e-mail]>
Acked-by: Rob Herring (Arm) <[e-mail]>
Signed-off-by: Janne Grunau <[e-mail]>

Documentation/devicetree/bindings/cpufreq/apple,cluster-cpufreq.yaml

···
           - const: apple,t7000-cluster-cpufreq
           - const: apple,s5l8960x-cluster-cpufreq
       - const: apple,s5l8960x-cluster-cpufreq
+      - items:
+          - const: apple,t6020-cluster-cpufreq
+          - const: apple,t8112-cluster-cpufreq
 
   reg:
     maxItems: 1