riscv: Kconfig: Enable cpufreq kconfig menu

Enable cpufreq kconfig menu for RISC-V.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20221115105135.1180490-2-prabhakar.mahadev-lad.rj@bp.renesas.com


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