feat(el3-spmc): make platform logical partition optional

Prior to this commit a logical platform specific partition is added when
compiling with SPMC at EL3. Not all platform need to add a logical
platform so make this optional.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Change-Id: I3bdd2a91350330c1637e8d84765974bfb6b225d7
diff --git a/services/std_svc/spm/el3_spmc/spmc.mk b/services/std_svc/spm/el3_spmc/spmc.mk
index c674e71..6442af0 100644
--- a/services/std_svc/spm/el3_spmc/spmc.mk
+++ b/services/std_svc/spm/el3_spmc/spmc.mk
@@ -20,7 +20,9 @@
                     ${PLAT}_el3_spmc_logical_sp.c)
 
 
+ifneq ($(wildcard $(SPMC_LP_SOURCES)),)
 SPMC_SOURCES += $(SPMC_LP_SOURCES)
+endif
 
 # Let the top-level Makefile know that we intend to include a BL32 image
 NEED_BL32		:=	yes