imx8: implement mmc_get_env_dev

Implement mmc_get_env_dev for i.MX8.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Anatolij Gustschin <agust@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
1 file changed