The use of AMU counters at the highest implemented exception level
can expose information about them to lower exception levels, such as
specific behavior happening in the CPU (e.g.: MPMM gear shifting in
TC2). In order to prevent this, read accesses to AMU counters are
restricted by default, so they are RAZ (read-as-zero) from lower
exception levels from now on.
Change-Id: I660b0928bea3fe09436ad53b0bb43c3067523178
Signed-off-by: Juan Pablo Conde <juanpablo.conde@arm.com>