dt-bindings: arm-pmu: Document Apple PMU compatible strings

As we are about to add support fur the Apple PMUs, document the compatible
strings associated with the two micro-architectures present in the Apple M1.

Acked-by: Rob Herring <[e-mail]>
Reviewed-by: Hector Martin <[e-mail]>
Signed-off-by: Marc Zyngier <[e-mail]>

Documentation/devicetree/bindings/arm/pmu.yaml

···
     items:
       - enum:
           - apm,potenza-pmu
+          - apple,firestorm-pmu
+          - apple,icestorm-pmu
           - arm,armv8-pmuv3 # Only for s/w models
           - arm,arm1136-pmu
           - arm,arm1176-pmu