dt-bindings: loongarch: Add Loongson SoC boards compatibles

Add Loongson SoC boards binding with DT schema format using json-schema.
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

Documentation/devicetree/bindings/loongarch/loongson.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/loongarch/loongson.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Loongson SoC-based boards
+
+maintainers:
+  - Binbin Zhou <zhoubinbin@loongson.cn>
+
+properties:
+  $nodename:
+    const: '/'
+  compatible:
+    oneOf:
+      - description: Loongson-2K0500 processor based boards
+        items:
+          - const: loongson,ls2k0500-ref
+          - const: loongson,ls2k0500
+
+      - description: Loongson-2K1000 processor based boards
+        items:
+          - const: loongson,ls2k1000-ref
+          - const: loongson,ls2k1000
+
+      - description: Loongson-2K2000 processor based boards
+        items:
+          - const: loongson,ls2k2000-ref
+          - const: loongson,ls2k2000
+
+additionalProperties: true
+
+...