imx: mx6: implement board_mmc_get_env_dev

Implement board_mmc_get_env_dev for the boards.

Following is examples:
SD1/SD2/SD3: return devno;
SD2/SD3: return devno - 1;
SD2/SD4: if (devno == 2), return dev - 2; return dev - 1;

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
4 files changed