s2idle breaks on machines without cpuidle support

[Kazuki]

Hi everyone,

s2idle is blocked on machines without proper cpuidle support here
in kernel/sched/idle.c:

> if (cpuidle_not_available(drv, dev)) {
> 	tick_nohz_idle_stop_tick();

> 	default_idle_call();
> 	goto exit_idle;
> }

> /*
>  * Suspend-to-idle ("s2idle") is a system state in which all user space
>  * has been frozen, all I/O devices have been suspended and the only

However, there are 2 problems with this approach:

1. The suspend framework does not expect this, and continues to suspend the
machine, which causes machines without proper cpuidle support to break when
suspending
2. Suspend actually works on ARM64 machines even without proper
cpuidle (PSCI cpuidle) since they support wfi, so the assumption here is wrong
on such machines

I'm not exactly sure how to figure this out, and my attempts have all led to an
unbootable kernel, so I've cc'ed the relevant people and hopefully we can find a
solution to this problem.

Thanks,
Kazuki