x86: link: Add a config for 64-bit U-Boot

Add a new board config which uses 64-bit U-Boot. This is not fully
functional but is it a start. Missing features:

- SDRAM sizing
- Booting linux
- EFI support
- SCSI device init
(and others)

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