Documentation: arm64: Document the PMU event counting threshold feature

Documentation: arm64: Document the PMU event counting threshold feature

Add documentation for the new Perf event open parameters and
the threshold_max capability file.

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: James Clark <james.clark@arm.com>
Link: https://lore.kernel.org/r/20231211161331.1277825-12-james.clark@arm.com
Signed-off-by: Will Deacon <will@kernel.org>