stm32mp: dram_init: Limit DDR usage under 4GB boundary for STM32MP

Limit DDR usage under 4GB boundary on STM32MP regardless of
memory size declared in device tree.

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