RISC-V: Enable CPU_IDLE drivers

We force select CPU_PM and provide asm/cpuidle.h so that we can
use CPU IDLE drivers for Linux RISC-V kernel.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Anup Patel <apatel@vetanamicro.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>