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: NPatrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: NPatrice Chotard <patrice.chotard@foss.st.com>

arch/arm/mach-stm32mp/dram_init.c

@@ -46,6 +46,9 @@ ulong board_get_usable_ram_top(ulong total_size)
 	phys_addr_t reg;
 	struct lmb lmb;
 
+	if (!total_size)
+		return gd->ram_base + gd->ram_size;
+
 	/* found enough not-reserved memory to relocated U-Boot */
 	lmb_init(&lmb);
 	lmb_add(&lmb, gd->ram_base, gd->ram_size);