env: mmc: allow support of mmc_get_env_dev with OF_CONTROL

Use the weak function mmc_get_env_dev in mmc_offset_try_partition
function to allow dynamic selection of mmc device to use
and no more use directly the define CONFIG_SYS_MMC_ENV_DEV.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>