MIPS: Loongson: Use CONFIG_NR_CPUS_DEFAULT_64 to support more CPUs

When I update the mainline kernel on the Loongson 2-way platform which
has 8 CPUs, it only shows 4 CPUs due to NR_CPUS is 4, this is obviously
wrong.

In order to support more CPUs on the Loongson platform, it is better
to use CONFIG_NR_CPUS_DEFAULT_64 instead of CONFIG_NR_CPUS_DEFAULT_4
to specify the maximum number of CPUs which the kernel will support.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Acked-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>