riscv: dts: change TH1520 files to dual license

Modify the SPDX-License-Identifier for dual license of GPL-2.0 OR MIT.
Signed-off-by: Drew Fustini <dfustini@baylibre.com>
Acked-by: Jisheng Zhang <jszhang@kernel.org>
Acked-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

arch/riscv/boot/dts/thead/th1520-lichee-module-4a.dtsi

-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: (GPL-2.0 OR MIT)
 /*
  * Copyright (C) 2023 Jisheng Zhang <jszhang@kernel.org>
  */

arch/riscv/boot/dts/thead/th1520-lichee-pi-4a.dts

-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: (GPL-2.0 OR MIT)
 /*
  * Copyright (C) 2023 Jisheng Zhang <jszhang@kernel.org>
  */

arch/riscv/boot/dts/thead/th1520.dtsi

-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: (GPL-2.0 OR MIT)
 /*
  * Copyright (C) 2021 Alibaba Group Holding Limited.
  * Copyright (C) 2023 Jisheng Zhang <jszhang@kernel.org>