imx: imx8mn-evk: convert to OF_UPSTREAM

Convert i.MX8MN-EVK and i.MX8MN-DDR4-EVK to OF_UPSTREAM

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/configs/imx8mn_ddr4_evk_defconfig b/configs/imx8mn_ddr4_evk_defconfig
index 743dea5..56b8880 100644
--- a/configs/imx8mn_ddr4_evk_defconfig
+++ b/configs/imx8mn_ddr4_evk_defconfig
@@ -8,7 +8,7 @@
 CONFIG_ENV_SIZE=0x1000
 CONFIG_ENV_OFFSET=0x400000
 CONFIG_DM_GPIO=y
-CONFIG_DEFAULT_DEVICE_TREE="imx8mn-ddr4-evk"
+CONFIG_DEFAULT_DEVICE_TREE="freescale/imx8mn-ddr4-evk"
 CONFIG_SPL_TEXT_BASE=0x912000
 CONFIG_TARGET_IMX8MN_DDR4_EVK=y
 CONFIG_SYS_MONITOR_LEN=524288