riscv: add SpacemiT SoC family Kconfig support

riscv: add SpacemiT SoC family Kconfig support

The first SoC in the SpacemiT series is K1, which contains 8 RISC-V
cores with RISC-V Vector v1.0 support.

Link: https://www.spacemit.com/en/spacemit-key-stone-2/
Signed-off-by: Yangyu Chen <cyy@cyyself.name>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Yixun Lan <dlan@gentoo.org>