MIPS: XLR, XLS: Add comment for smp setup

It seems that BSP could be setup twice, but the nlm_cpu_ready array is only
set for ASPs in smpboot.S, not including BSP.

Signed-off-by: Hillf Danton <dhillf@gmail.com>
Cc: "Jayachandran C." <jayachandranc@netlogicmicro.com>
Cc: LKML <linux-kernel@vger.kernel.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2695/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>,