imx: imx8mm_evk: add board_mmc_get_env_dev

Add board_mmc_get_env_dev to correctly return devno for mmc env

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/board/freescale/imx8mm_evk/imx8mm_evk.c b/board/freescale/imx8mm_evk/imx8mm_evk.c
index 3706e1e..e474233 100644
--- a/board/freescale/imx8mm_evk/imx8mm_evk.c
+++ b/board/freescale/imx8mm_evk/imx8mm_evk.c
@@ -19,6 +19,11 @@
 	return 0;
 }
 
+int board_mmc_get_env_dev(int devno)
+{
+	return devno;
+}
+
 int board_late_init(void)
 {
 #ifdef CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG