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>