configs: stm32mp: activate CONFIG_ENV_MMC_USE_DT

Activate by default CONFIG_ENV_MMC_USE_DT as "u-boot,mmc-env-partition"
should be always use in STMicroelectronics boards device tree to locate
the environment for mmc backend. The 2 defines:
  CONFIG_ENV_OFFSET=0x280000
  CONFIG_ENV_OFFSET_REDUND=0x2C0000
are only valid for spi-nor and not for SD-Card or eMMC.

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