board: stm32mp1: solve compilation issue when ENV_IS_IN_MMC is deactivated

Solve compilation issue on undefined CONFIG_SYS_MMC_ENV_DEV when
CONFIG_ENV_IS_IN_MMC is deactivated on STMicroelectronics boards
defconfig

Fixes: 9f971936 ("board: stm32mp1: use CONFIG_SYS_MMC_ENV_DEV when available")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>