x86: Add comments to the SIPI vector

The Intel SIPI (start-up inter-processor interrupt) vector is the entry
point for each secondary CPU (also called an AP - applications processor).
The assembler and C code are linked, so add comments to indicate this.

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