dt-bindings: add ARMv8 PMU binding

Add missing "arm,armv8-pmuv3" compatible property for ARMv8 PMU.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Pawel Moll <pawel.moll@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>