Implement mmc_get_env_dev, devno can be got from smbr1 of SRC.
Introduce a weak function board_mmc_get_env_dev, different
boards can implement it according to different sdhc controllers
that used by the board.

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