dt-bindings: riscv: Add SpacemiT X60 compatibles

The X60 is RISC-V CPU cores from SpacemiT and currently used in their K1
SoC.

Link: https://www.spacemit.com/en/spacemit-x60-core/
Signed-off-by: Yangyu Chen <cyy@cyyself.name>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Yixun Lan <dlan@gentoo.org>

Author: cyyself

Documentation/devicetree/bindings/riscv/cpus.yaml

@@ -46,6 +46,7 @@ properties:
               - sifive,u7
               - sifive,u74
               - sifive,u74-mc
+              - spacemit,x60
               - thead,c906
               - thead,c908
               - thead,c910