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>

arch/x86/include/asm/sipi.h

@@ -42,6 +42,7 @@ struct __packed sipi_params_16bit {
  * struct sipi_params - 32-bit SIP entry-point parameters
  *
  * These are used by the AP init code and must be set up before the APs start.
+ * The members must match with the sipi_params layout in sipi_vector.S.
  *
  * The stack area extends down from @stack_top, with @stack_size allocated
  * for each AP.