imx: spl: support i.MX8MP spl_boot_device

i.MX8MP follows i.MX8MN, so just let it 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 dde1635..5a6493a6 100644
--- a/arch/arm/mach-imx/spl.c
+++ b/arch/arm/mach-imx/spl.c
@@ -135,7 +135,8 @@
 
 	enum boot_device boot_device_spl = get_boot_device();
 
-	if (IS_ENABLED(CONFIG_IMX8MM) || IS_ENABLED(CONFIG_IMX8MN))
+	if (IS_ENABLED(CONFIG_IMX8MM) || IS_ENABLED(CONFIG_IMX8MN) ||
+	    IS_ENABLED(CONFIG_IMX8MP))
 		return spl_board_boot_device(boot_device_spl);
 
 	switch (boot_device_spl) {