imx: spl: use spl_board_boot_device for i.MX8MN

i.MX8MN follow same logic as i.MX8MM, so use spl_board_boot_device

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/arch/arm/mach-imx/spl.c b/arch/arm/mach-imx/spl.c
index 5cc74b6..dde1635 100644
--- a/arch/arm/mach-imx/spl.c
+++ b/arch/arm/mach-imx/spl.c
@@ -135,7 +135,7 @@
 
 	enum boot_device boot_device_spl = get_boot_device();
 
-	if (IS_ENABLED(CONFIG_IMX8MM))
+	if (IS_ENABLED(CONFIG_IMX8MM) || IS_ENABLED(CONFIG_IMX8MN))
 		return spl_board_boot_device(boot_device_spl);
 
 	switch (boot_device_spl) {