x86: spl: Make moving BSS conditional

At present BSS is always placed in SDRAM. If a separate BSS is not in use
this means that BSS doesn't work as expected. Make the setting conditional
on the SEPARATE_BSS option.

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