feat(imx8mn): enable dram retention suuport on imx8mn

Enable dram retention support on i.MX8MN.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Change-Id: I9b3a08efbbd154b2fc7e41bedb36a4d4e3784448
diff --git a/plat/imx/imx8m/imx8mn/platform.mk b/plat/imx/imx8m/imx8mn/platform.mk
index 54be41b..5ffce44 100644
--- a/plat/imx/imx8m/imx8mn/platform.mk
+++ b/plat/imx/imx8m/imx8mn/platform.mk
@@ -13,6 +13,10 @@
 # Include GICv3 driver files
 include drivers/arm/gic/v3/gicv3.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		\
@@ -36,6 +40,7 @@
 				drivers/arm/tzc/tzc380.c			\
 				drivers/delay_timer/delay_timer.c		\
 				drivers/delay_timer/generic_delay_timer.c	\
+				${IMX_DRAM_SOURCES}				\
 				${IMX_GIC_SOURCES}				\
 				${XLAT_TABLES_LIB_SRCS}