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>
3 files changed