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>

Author: cyyself

arch/riscv/Kconfig.socs

@@ -24,6 +24,11 @@ config ARCH_SOPHGO
 	help
 	  This enables support for Sophgo SoC platform hardware.
 
+config ARCH_SPACEMIT
+	bool "SpacemiT SoCs"
+	help
+	  This enables support for SpacemiT SoC platform hardware.
+
 config ARCH_STARFIVE
 	def_bool SOC_STARFIVE