stm32mp1: ram: add tuning in DDR interactive mode

Add command tuning for DDR interactive mode, used during
board bring-up or with CubeMX DDR tools to execute software
tuning for the DDR configuration:
- software read DQS Gating (replace the built-in one)
- Bit de-skew
- Eye Training or DQS training

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>