driver: ddr: imx: skip ddr_ss_gpr config on imx8mn

There is no DDR_SS_GPR0 exits on i.MX8MN, so skip setting
this register on i.MX8MN.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/drivers/ddr/imx/imx8m/ddr_init.c b/drivers/ddr/imx/imx8m/ddr_init.c
index bbddee6..1c5c7f9 100644
--- a/drivers/ddr/imx/imx8m/ddr_init.c
+++ b/drivers/ddr/imx/imx8m/ddr_init.c
@@ -74,7 +74,7 @@
 
 	/* if ddr type is LPDDR4, do it */
 	tmp = reg32_read(DDRC_MSTR(0));
-	if (tmp & (0x1 << 5))
+	if (tmp & (0x1 << 5) && !is_imx8mn())
 		reg32_write(DDRC_DDR_SS_GPR0, 0x01); /* LPDDR4 mode */
 
 	/* determine the initial boot frequency */