dt-bindings/perf: Add Arm CoreSight PMU

Add a binding for implementations of the Arm CoreSight Performance
Monitoring Unit Architecture. Not to be confused with CoreSight debug
and trace, the PMU architecture defines a standard MMIO interface for
event counters following a similar design to the CPU PMU architecture,
where the implementation and most of its features are discoverable
through ID registers.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/c62a86ef177bec5c6d12176c605de900e9e40c87.1706718007.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>