stm32mp1: ram: remove tuning support

Remove the DDR interactive command tuning, as the support of a predefined
DDR PHY tuning is removed for STM32MP1 driver in SPL and in TF-A
and the result of this tuning will be never used.

Moreover this SW tuning procedure can failed on some hardware
configuration (to many BIST errors and no convergence); it will be no
more supported in the next delivery of the DDR utilities included in
the CubeMX tool of STMicroelectronics.

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