MIPS: Support hard limit of cpu count (nr_cpu_ids)

On MIPS currently, only the soft limit of cpu count (maxcpus) has its
effect, this patch enable the hard limit (nr_cpus) as well. Processor
cores which greater than maxcpus and less than nr_cpus can be taken up
via cpu hotplug. The code is borrowed from X86.

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Reviewed-by: Andreas Herrmann <andreas.herrmann@caviumnetworks.com>
Cc: John Crispin <john@phrozen.org>
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: linux-mips@linux-mips.org
Cc: Fuxin Zhang <zhangfx@lemote.com>
Cc: Zhangjin Wu <wuzhangjin@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/7183/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>