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 <robh@kernel.org>
Reviewed-by: Hector Martin <marcan@marcan.st>
Signed-off-by: Marc Zyngier <maz@kernel.org>

Documentation/devicetree/bindings/arm/pmu.yaml

@@ -20,6 +20,8 @@ properties:
     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