plat: imx: fix boot hang on imx8mm ddr4 board

skip init the dram info if the ddr type is DDR4,
support for it will be added later.

Signed-off-by: Bai Ping <ping.bai@nxp.com>
(cherry picked from commit e88e844028da9805c8de695ef2371090e44ccfe2)
diff --git a/plat/imx/common/imx8m/dram.c b/plat/imx/common/imx8m/dram.c
index e5a40c8..cb987c4 100644
--- a/plat/imx/common/imx8m/dram.c
+++ b/plat/imx/common/imx8m/dram.c
@@ -64,6 +64,9 @@
 	if (ddr_type == DDRC_LPDDR4) {
 		dram_info = &imx8m_lpddr4_dram_info;
 		dram_info->dram_type = ddr_type;
+	} else {
+		/* TODO DDR4 support will be added later */
+		return;
 	}
 
 	/* init the boot_fsp & current_fsp */