x86: Tidy up address for loading U-Boot from SPL

Use the binman symbols for this, to avoid hard-coding the value. We could
use CONFIG_X86_OFFSET_U_BOOT for the address, but it seems better to
obtain the offset and size through the same mechanism.

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