i.MX6UL: isiot: Add mmc_late_init

Let the runtime code can set the mmcdev and mmcroot based
on the devno using mmc_get_env_dev instead of defining
separately in build-time configs using mmc_late_init func.

Cc: Matteo Lisi <matteo.lisi@engicam.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Reviewed by: Stefano Babic <sbabic@denx.de>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>