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>