imx: mx7dsabresd: move mmc_get_env_devno to soc code

Move mmc_get_env_devno to soc.c and rename to mmc_get_env_dev to
match the one in common/env_mmc.c.
Introduce a weak function board_mmc_get_env_dev. Different
boards can implement this according to sdhc controller which
is used by the board.

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