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>

Documentation/devicetree/bindings/mips/loongson/devices.yaml

@@ -32,4 +32,8 @@ properties:
       - description: Generic Loongson64 Quad Core + LS7A
         items:
           - const: loongson,loongson64g-4core-ls7a
+
+      - description: Virtual Loongson64 Quad Core + VirtIO
+        items:
+          - const: loongson,loongson64v-4core-virtio
 ...