arm64: Support hard limit of cpu count by nr_cpus

Enable the hard limit of cpu count by set boot options nr_cpus=x
on arm64, and make a minor change about message when total number
of cpu exceeds the limit.

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Reported-by: Shiyuan Hu <hushiyuan@huawei.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>