cpuidle: RISC-V: Move few functions to arch/riscv

cpuidle: RISC-V: Move few functions to arch/riscv

To support ACPI Low Power Idle (LPI), few functions are required which
are currently static functions in the DT based cpuidle driver. Hence,
move them under arch/riscv so that ACPI driver also can use them. Since
they are no longer static functions, append "riscv_" prefix to the
function name.

Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/20240118062930.245937-2-sunilvl@ventanamicro.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>