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: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>