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: 92b611e8b003 ("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>
diff --git a/arch/arm/mach-stm32mp/dram_init.c b/arch/arm/mach-stm32mp/dram_init.c
index 94f25f3..920b99b 100644
--- a/arch/arm/mach-stm32mp/dram_init.c
+++ b/arch/arm/mach-stm32mp/dram_init.c
@@ -47,7 +47,7 @@
 	struct lmb lmb;
 
 	if (!total_size)
-		return gd->ram_base + gd->ram_size;
+		return gd->ram_top;
 
 	/* found enough not-reserved memory to relocated U-Boot */
 	lmb_init(&lmb);