MIPS: Update default config file for Loongson-3

Update Loongson-3's default config file:
1, Adjust NR_CPUS to 16;
2, Add a built-in cmdline "ieee754=relaxed";
3, Enable MSA, CGROUPS, NAMESPACES, KVM, and XFS support;
4, Enable all possible virtio drivers to support KVM Host/Guest;
5, Enable all necessary netfilter modules to support virtual network;

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>