env_mmc: Allow SPL to use any MMC device to load/save the environment

env_mmc: Allow SPL to use any MMC device to load/save the environment

SPL has been restricted to use only dev 0 based on the assumption that only
one MMC device is registered. This is not always the case and many
platforms now register several devices as expected by the spl mmc boot code
For those platform SPL_ENV_SUPPORT is broken if dev is forced to 0.

A word of warning: this commit may break SPL_ENV_SUPPORT on platforms that
do not register the same MMC controllers in SPL and in u-boot (mostly iMX6
based platforms). Fortunately none of those activate SPL_ENV_SUPPORT in
their default configuration.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>