env: mmc: add CONFIG_ENV_MMC_USE_DT

Add a new config CONFIG_ENV_MMC_USE_DT to force configuration of the
U-Boot environment offset with device tree config node.

This patch avoids issues when several CONFIG_ENV_IS_IN_XXX are activated,
the defconfig file uses the same value for CONFIG_ENV_OFFSET or
CONFIG_ENV_OFFSET_REDUND for the several ENV backends (SPI_FLASH, EEPROM
NAND, SATA, MMC).

After this patch a bad offset value is not possible when the selected
partition in device tree is not found.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>