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

Signed-off-by: Ionela Voinescu
Reviewed-by: Valentin Schneider
Cc: Jonathan Corbet
Cc: Will Deacon
Cc: Catalin Marinas
Signed-off-by: Catalin Marinas