ram: stm32mp1: compute DDR size from DDRCTL registers

Compute the DDR size from DDR controller register (mstr and addrmap)
in U-Boot proper as the DDR information are useful only for SPL
but not for U-Boot proper, for example with TFABOOT.

This patch simplify U-Boot DT when several DDR size are supported
and support of next SOC in STM32MP family.

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