dt-bindings: riscv: Add T-HEAD TH1520 board compatibles

Several SoMs and boards are available that feature the T-HEAD TH1520
SoC. Document the compatible strings.
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

Documentation/devicetree/bindings/riscv/thead.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/riscv/thead.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: T-HEAD SoC-based boards
+
+maintainers:
+  - Jisheng Zhang <jszhang@kernel.org>
+
+description:
+  T-HEAD SoC-based boards
+
+properties:
+  $nodename:
+    const: '/'
+  compatible:
+    oneOf:
+      - description: Sipeed Lichee Pi 4A board for the Sipeed Lichee Module 4A
+        items:
+          - enum:
+              - sipeed,lichee-pi-4a
+          - const: sipeed,lichee-module-4a
+          - const: thead,th1520
+
+additionalProperties: true
+
+...