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/board/freescale/imx8mn_evk/Kconfig b/board/freescale/imx8mn_evk/Kconfig
index 38ac846..048fb7d 100644
--- a/board/freescale/imx8mn_evk/Kconfig
+++ b/board/freescale/imx8mn_evk/Kconfig
@@ -1,4 +1,4 @@
-if TARGET_IMX8MN_EVK
+if TARGET_IMX8MN_DDR4_EVK
 
 config SYS_BOARD
 	default "imx8mn_evk"