feat(imx8mm): enable dram retention suuport on imx8mm

Enable dram retention support on i.MX8MM.

Change-Id: I76ada615d386602e551d572ff4e60ee19bb8e418
Signed-off-by: Jacky Bai <ping.bai@nxp.com>
diff --git a/plat/imx/imx8m/imx8mm/platform.mk b/plat/imx/imx8m/imx8mm/platform.mk
index 0cce7ca..df04997 100644
--- a/plat/imx/imx8m/imx8mm/platform.mk
+++ b/plat/imx/imx8m/imx8mm/platform.mk
@@ -19,6 +19,9 @@
 
 include lib/libfdt/libfdt.mk
 
+IMX_DRAM_SOURCES	:=	plat/imx/imx8m/ddr/dram.c		\
+				plat/imx/imx8m/ddr/dram_retention.c
+
 IMX_GIC_SOURCES		:=	${GICV3_SOURCES}			\
 				plat/common/plat_gicv3.c		\
 				plat/common/plat_psci_common.c		\
@@ -43,6 +46,7 @@
 				drivers/delay_timer/delay_timer.c		\
 				drivers/delay_timer/generic_delay_timer.c	\
 				${XLAT_TABLES_LIB_SRCS}				\
+				${IMX_DRAM_SOURCES}				\
 				${IMX_GIC_SOURCES}
 
 ifeq (${NEED_BL2},yes)