dt-bindings: riscv: Add T-HEAD C920 compatibles

The C920 is RISC-V CPU cores from T-HEAD Semiconductor.
Notably, the C920 core is used in the SOPHGO's SG2042 SoC.

Acked-by: Chao Wei <chao.wei@sophgo.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Chen Wang <unicorn_wang@outlook.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>