imx: imx8mn_evk: add board_mmc_get_env_dev

Add board_mmc_get_env_dev, otherwise,
Loading Environment from MMC...
MMC Device 0 not found
*** Warning - No MMC card found, using default environment
Reviewed-by: NFabio Estevam <festevam@gmail.com>
Signed-off-by: NPeng Fan <peng.fan@nxp.com>

board/freescale/imx8mn_evk/imx8mn_evk.c

@@ -19,6 +19,11 @@ int board_init(void)
 	return 0;
 }
 
+int board_mmc_get_env_dev(int devno)
+{
+	return devno;
+}
+
 int board_late_init(void)
 {
 #ifdef CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG