dt-bindings: mips: Document Loongson kvm guest board

Document loongson64v-4core-virtio, a virtio based kvm guest board for
Loongson-3.

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