plat/arm/sgi: refactor the inclusion of memory mapping

Upcoming RD platforms have a different memory map from those of the
existing platforms. So make the build of the existing mmap entries to be
usable only for existing platforms and let upcoming platforms define
a different set of mmap entries.

Change-Id: Id1ef0293efe8749c78a99237e78d32573c7233aa
Signed-off-by: Aditya Angadi <aditya.angadi@arm.com>
diff --git a/plat/arm/board/rdn1edge/platform.mk b/plat/arm/board/rdn1edge/platform.mk
index b313426..d65854f 100644
--- a/plat/arm/board/rdn1edge/platform.mk
+++ b/plat/arm/board/rdn1edge/platform.mk
@@ -15,6 +15,8 @@
 
 SGI_CPU_SOURCES		:=	lib/cpus/aarch64/neoverse_n1.S
 
+PLAT_BL_COMMON_SOURCES	+=	${CSS_ENT_BASE}/sgi_plat.c
+
 BL1_SOURCES		+=	${SGI_CPU_SOURCES}			\
 				${RDN1EDGE_BASE}/rdn1edge_err.c