Implement support for the Activity Monitor Unit on Cortex A75

The Cortex A75 has 5 AMU counters.  The first three counters are fixed
and the remaining two are programmable.

A new build option is introduced, `ENABLE_AMU`.  When set, the fixed
counters will be enabled for use by lower ELs.  The programmable
counters are currently disabled.

Change-Id: I4bd5208799bb9ed7d2596e8b0bfc87abbbe18740
Signed-off-by: Dimitris Papastamos <dimitris.papastamos@arm.com>
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index b7ce051..9f7abed 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -156,3 +156,5 @@
 ifeq (${ARCH},aarch32)
     override ENABLE_SPE_FOR_LOWER_ELS := 0
 endif
+
+ENABLE_AMU			:= 0