stm32mp1: stm32prog: remove stm32prog_get_tee_partitions with FIP

The MTD tee partitions used to save the OP-TEE binary are needed when
TF-A doesn't use the FIP container to load binaries.

This patch puts under CONFIG_STM32MP15x_STM32IMAGE flag the associated
code in U-Boot binary and prepare the code cleanup when
CONFIG_STM32MP15x_STM32IMAGE support will be removed after TF-A migration
to FIP support.

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