feat(neoverse): enable NEOVERSE_Nx_EXTERNAL_LLC flag

Neoverse reference design platforms include a system level cache in the
interconnect and that is the last level cache. So enable the build flag
'NEOVERSE_Nx_EXTERNAL_LLC' for all the Neoverse reference design
platforms.

Change-Id: I813b3ef7ea7dc4e335b44a88e019d8c56f05f4ac
Signed-off-by: Nuno Lopes <nuno.lopes@arm.com>
diff --git a/plat/arm/board/rdn1edge/platform.mk b/plat/arm/board/rdn1edge/platform.mk
index 95753aa..d8d1293 100644
--- a/plat/arm/board/rdn1edge/platform.mk
+++ b/plat/arm/board/rdn1edge/platform.mk
@@ -73,3 +73,6 @@
 endif
 
 override CTX_INCLUDE_AARCH32_REGS	:= 0
+
+# Enable the flag since RD-N1-EDGE has a system level cache
+NEOVERSE_Nx_EXTERNAL_LLC		:=	1