The interaction between the exclude_{host,guest} flags,
exclude_{user,kernel,hv} flags and presence of VHE can result in
different exception levels being filtered by the ARMv8 PMU. As this
can be confusing let's document how they work on arm64.
Signed-off-by: Andrew Murray <andrew.murray@arm.com> Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com> Acked-by: Will Deacon <will.deacon@arm.com> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>