dt-bindings: ARM: Clean up PMU compatible list

The "alpha by vendor, reverse-alpha by model" sorting of compatibles
that we seem to have ended up with is decidedly odd. Make it less so.

Also copy the comment from the generic "arm,armv8" CPU binding to help
clarify that the "arm,armv8-pmuv3" binding is rather intended to be a
counterpart to that, for describing implementations without a specific
microarchitecture like the AEMv8 software model.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>