feat(intel): enable SDMMC frontdoor load for ATF->Linux

SDMMC is 1 of the boot source for Agilex5 and legacy products.
By enabling this, ATF is able to read out the DTB binary and
loaded it to DDR for Linux boot.

Change-Id: Ida303fb43ea63013a08083ce65952c5ad4e28f93
Signed-off-by: Jit Loon Lim <jit.loon.lim@intel.com>
Signed-off-by: Sieu Mun Tang <sieu.mun.tang@intel.com>
diff --git a/plat/intel/soc/n5x/platform.mk b/plat/intel/soc/n5x/platform.mk
index 95f076f..29fd0eb 100644
--- a/plat/intel/soc/n5x/platform.mk
+++ b/plat/intel/soc/n5x/platform.mk
@@ -46,6 +46,8 @@
 		plat/intel/soc/common/soc/socfpga_mailbox.c		\
 		plat/intel/soc/common/soc/socfpga_reset_manager.c
 
+$(eval $(call add_define,ARM_PRELOADED_DTB_BASE))
+
 PROGRAMMABLE_RESET_ADDRESS	:= 0
 RESET_TO_BL2			:= 1
 BL2_INV_DCACHE			:= 0