imx: imx8mn_ddr4_evk: Use CONFIG_TARGET_IMX8MN_DDR4_EVK for DDR4 EVK board

use CONFIG_TARGET_IMX8MN_DDR4_EVK for DDR4 EVK board, we will use
CONFIG_TARGET_IMX8MN_EVK for LPDDR4 EVK board.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/arch/arm/mach-imx/imx8m/Kconfig b/arch/arm/mach-imx/imx8m/Kconfig
index 8615dc3..bd2ab77 100644
--- a/arch/arm/mach-imx/imx8m/Kconfig
+++ b/arch/arm/mach-imx/imx8m/Kconfig
@@ -43,7 +43,7 @@
 	select SUPPORT_SPL
 	select IMX8M_LPDDR4
 
-config TARGET_IMX8MN_EVK
+config TARGET_IMX8MN_DDR4_EVK
 	bool "imx8mn DDR4 EVK board"
 	select IMX8MN
 	select SUPPORT_SPL