dt-bindings: Document Loongson PCI Host Controller

PCI host controller found on Loongson PCHs and SoCs.
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

Documentation/devicetree/bindings/pci/loongson.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/pci/loongson.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Loongson PCI Host Controller
+
+maintainers:
+  - Jiaxun Yang <jiaxun.yang@flygoat.com>
+
+description: |+
+  PCI host controller found on Loongson PCHs and SoCs.
+
+allOf:
+  - $ref: /schemas/pci/pci-bus.yaml#
+
+properties:
+  compatible:
+    oneOf:
+      - const: loongson,ls2k-pci
+      - const: loongson,ls7a-pci
+      - const: loongson,rs780e-pci
+
+  reg:
+    minItems: 1
+    maxItems: 2
+    items:
+      - description: CFG0 standard config space register
+      - description: CFG1 extended config space register
+
+  ranges:
+    minItems: 1
+    maxItems: 3
+
+
+required:
+  - compatible
+  - reg
+  - ranges
+
+examples:
+  - |
+
+    bus {
+        #address-cells = <2>;
+        #size-cells = <2>;
+        pcie@1a000000 {
+            compatible = "loongson,rs780e-pci";
+            device_type = "pci";
+            #address-cells = <3>;
+            #size-cells = <2>;
+
+            // CPU_PHYSICAL(2)  SIZE(2)
+            reg = <0x0 0x1a000000  0x0 0x2000000>;
+
+            // BUS_ADDRESS(3)  CPU_PHYSICAL(2)  SIZE(2)
+            ranges = <0x01000000 0x0 0x00004000  0x0 0x00004000  0x0 0x00004000>,
+                     <0x02000000 0x0 0x40000000  0x0 0x40000000  0x0 0x40000000>;
+        };
+    };
+...