UPSTREAM: Documentation: arm64: document support for the AMU extension

The activity monitors extension is an optional extension introduced
by the ARMv8.4 CPU architecture.

Add initial documentation for the AMUv1 extension:
 - arm64/amu.txt: AMUv1 documentation
 - arm64/booting.txt: system registers initialisation

Bug: 172781912
Change-Id: I1a892d7aa98ff545a3fe326f5a827337f41ddf19
Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
(cherry picked from commit 6abde90881a5ea3a383c0959fdd7f575f95db4b3)
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>