refactor(st-ddr): reorganize generic and specific elements

stm32mp_ddrctl structure contains DDRCTRL registers definitions.
stm32mp_ddr_info contains general DDR information extracted from DT.
stm32mp_ddr_size moves to the generic side.
stm32mp1_ddr_priv contains platform private data.

stm32mp_ddr_dt_get_info() and stm32mp_ddr_dt_get_param() allow to
retrieve data from DT. They are located in new generic c/h files in
which stm32mp_ddr_param structure is declared. Platform makefile
is updated.

Adapt driver with this new classification.

Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@st.com>
Change-Id: I4187376c9fff1a30e7a94407d188391547107997