cpuidle: Add RISC-V SBI CPU idle driver

The RISC-V SBI HSM extension provides HSM suspend call which can
be used by Linux RISC-V to enter platform specific low-power state.

This patch adds a CPU idle driver based on RISC-V SBI calls which
will populate idle states from device tree and use SBI calls to
entry these idle states.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Acked-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>