AMU: Implement support for aarch64

The `ENABLE_AMU` build option can be used to enable the
architecturally defined AMU counters.  At present, there is no support
for the auxiliary counter group.

Change-Id: I7ea0c0a00327f463199d1b0a481f01dadb09d312
Signed-off-by: Dimitris Papastamos <dimitris.papastamos@arm.com>
diff --git a/bl31/bl31.mk b/bl31/bl31.mk
index fccdc8a..ebd0e71 100644
--- a/bl31/bl31.mk
+++ b/bl31/bl31.mk
@@ -50,6 +50,10 @@
 BL31_SOURCES		+=	lib/extensions/spe/spe.c
 endif
 
+ifeq (${ENABLE_AMU},1)
+BL31_SOURCES		+=	lib/extensions/amu/aarch64/amu.c
+endif
+
 BL31_LINKERFILE		:=	bl31/bl31.ld.S
 
 # Flag used to indicate if Crash reporting via console should be included