dt-bindings: mips: Add loongson boards

Prepare for later dts.
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Co-developed-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

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

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mips/loongson/devices.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Loongson based Platforms Device Tree Bindings
+
+maintainers:
+  - Jiaxun Yang <jiaxun.yang@flygoat.com>
+description: |
+  Devices with a Loongson CPU shall have the following properties.
+
+properties:
+  $nodename:
+    const: '/'
+  compatible:
+    oneOf:
+
+      - description: Generic Loongson3 Quad Core + RS780E
+        items:
+          - const: loongson,loongson3-4core-rs780e
+
+      - description: Generic Loongson3 Octa Core + RS780E
+        items:
+          - const: loongson,loongson3-8core-rs780e
+...