efi_loader: fix efi_add_known_memory()

In efi_add_known_memory() we currently call board_get_usable_ram_top() with
an incorrect value 0 of parameter total_size. This leads to an incorrect
value for ram_top depending on the code in board_get_usable_ram_top().

Use the value of gd->ram_top instead which is set before relocation by
calling board_get_usable_ram_top().

Fixes: 7b78d643 ("efi_loader: Reserve unaccessible memory")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>