riscv: dts: add initial T-HEAD TH1520 SoC device tree

Add initial device tree for the TH1520 RISC-V SoC by T-HEAD.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

Author: xhackerustc