dt-bindings: riscv: Add T-HEAD C908 compatible

dt-bindings: riscv: Add T-HEAD C908 compatible

The thead,c908 is a RISC-V CPU core from T-HEAD Semiconductor which used
in Canaan Kendryte K230 SoC.

Signed-off-by: Yangyu Chen <cyy@cyyself.name>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/linux-riscv/tencent_22BA0425B4DF1CA1713B62E4423C1BFBF809@qq.com/
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>