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>

docs: arm64: Move arm64 documentation under Documentation/arch/

Architecture-specific documentation is being moved into Documentation/arch/
as a way of cleaning up the top-level documentation directory and making
the docs hierarchy more closely match the source hierarchy. Move
Documentation/arm64 into arch/ (along with the Chinese equvalent
translations) and fix up documentation references.

Cc: Will Deacon <will@kernel.org>
Cc: Alex Shi <alexs@kernel.org>
Cc: Hu Haowen <src.res@email.cn>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Yantengsi <siyanteng@loongson.cn>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>