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>