x86: efi_loader: Use efi_add_conventional_memory_map()

Use efi_add_conventional_memory_map() to configure EFI conventional memory
properly with ram_top value. This will give 32-bit mode U-Boot proper
conventional memory regions even if e820 has an entry which is greater than
32-bit address space.

Signed-off-by: Aiden Park <aiden.park@intel.com>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
[bmeng: fixed some typos in the commit message]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>