Documentation: arm64: Document PMU counters access from userspace

commit aa1005d1 upstream

Add documentation to describe the access to the pmu hardware counters from
userspace.

Signed-off-by: Raphael Gault <raphael.gault@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211208201124.310740-6-robh@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>

[AZ: backport and adjust context of aa1005d1 from master branch of linux-mainline]
Signed-off-by: Adrian Zaharia <Adrian.Zaharia@windriver.com>
Signed-off-by: Ruiqiang Hao <Ruiqiang.Hao@windriver.com>