stm32mp: correctly handle board_get_usable_ram_top(0)

The function board_get_usable_ram_top can be called after relocation
with total_size = 0 to get the uppermost pointer that is valid to access
in U-Boot.

When total_size = 0, the reserved memory should be not take in account
with lmb library and 'gd->ram_base + gd->ram_size' can be used.

It is the case today in lib/efi_loader/efi_memory.c:efi_add_known_memory()
and this patch avoids that the reserved memory for OP-TEE is not part of
the EFI available memory regions.

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