dt-bindings: loongarch: Add CPU bindings for LoongArch

Add the available CPUs in LoongArch 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/cpus.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/loongarch/cpus.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: LoongArch CPUs
+
+maintainers:
+  - Binbin Zhou <zhoubinbin@loongson.cn>
+
+description:
+  This document describes the list of LoongArch CPU cores that support FDT,
+  it describe the layout of CPUs in a system through the "cpus" node.
+
+allOf:
+  - $ref: /schemas/cpu.yaml#
+
+properties:
+  compatible:
+    enum:
+      - loongson,la264
+      - loongson,la364
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+  - clocks
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/loongson,ls2k-clk.h>
+
+    cpus {
+        #size-cells = <0>;
+        #address-cells = <1>;
+
+        cpu@0 {
+            compatible = "loongson,la264";
+            device_type = "cpu";
+            reg = <0>;
+            clocks = <&clk LOONGSON2_NODE_CLK>;
+        };
+
+        cpu@1 {
+            compatible = "loongson,la264";
+            device_type = "cpu";
+            reg = <1>;
+            clocks = <&clk LOONGSON2_NODE_CLK>;
+        };
+    };
+
+...