With EFI the start address of U-Boot is specified differently. We could
consider just setting GD_FLG_RELOC and then setting up reloc_off. But that
flag has other implementations and we are not able to use U-Boot relocation
which this flag implies.

Instead, just add a special case for EFI.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>