stm32mp: Fix board_get_usable_ram_top()

When booting in EFI, lib/efi_loader/efi_memory.c calls
board_get_usable_ram_top(0) which returns by default
gd->ram_base + gd->ram_size which is the top of DDR.

In case of OPTEE boot, the top of DDR is currently reserved by OPTEE,
board_get_usable_ram_top(0) must return an address outside OPTEE
reserved memory.

gd->ram_top matches this constraint as it has already been initialized
by substracting all DT reserved-memory (included OPTEE memory area).

Fixes: 92b611e8 ("stm32mp: correctly handle board_get_usable_ram_top(0)")

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