dt-bindings: riscv: Add T-HEAD C906 and C910 compatibles

The C906 and C910 are RISC-V CPU cores from T-HEAD Semiconductor.
Notably, the C906 core is used in the Allwinner D1 SoC.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>