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/ddr/dram_retention.c b/plat/imx/imx8m/ddr/dram_retention.c
index 73eb8c4..7d4f823 100644
--- a/plat/imx/imx8m/ddr/dram_retention.c
+++ b/plat/imx/imx8m/ddr/dram_retention.c
@@ -145,9 +145,13 @@
 
 	/* before write Dynamic reg, sw_done should be 0 */
 	mmio_write_32(DDRC_SWCTL(0), 0x0);
+
+#if !PLAT_imx8mn
 	if (dram_info.dram_type == DDRC_LPDDR4) {
 		mmio_write_32(DDRC_DDR_SS_GPR0, 0x01); /*LPDDR4 mode */
 	}
+#endif /* !PLAT_imx8mn */
+
 	mmio_write_32(DDRC_DFIMISC(0), 0x0);
 
 	/* dram phy re-init */