ACPI: RISC-V: Add LPI driver

Enable Low Power Idle (LPI) based cpuidle driver for RISC-V platforms.
It depends on SBI HSM calls for idle state transitions.

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/20240118062930.245937-3-sunilvl@ventanamicro.com


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