ACPI: RISC-V: Add CPPC driver

Add cpufreq driver based on ACPI CPPC for RISC-V. The driver uses either
SBI CPPC interfaces or the CSRs to access the CPPC registers as defined
by the RISC-V FFH spec.

Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Pierre Gondois <pierre.gondois@arm.com>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/20240208034414.22579-2-sunilvl@ventanamicro.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>