riscv: defconfig: Enable CONFIG_CLK_SOPHGO_CV1800

CONFIG_CLK_SOPHGO_CV1800 is required when booting the minimum
system for CV1800 series board.

Signed-off-by: Inochi Amaoto <inochiama@outlook.com>
Link: https://lore.kernel.org/r/IA1PR20MB49537E8B2D1FAAA7D5B8BDA2BB052@IA1PR20MB4953.namprd20.prod.outlook.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>