ARM: decouple CPU offlining from reboot/shutdown

Add comments to machine_shutdown()/halt()/power_off()/restart() that
describe their requirements re: CPUs being active or not.

Remove smp_kill_cpus(), and its call from smp_send_stop().
smp_kill_cpus() was indirectly calling smp_ops.cpu_kill() without calling
smp_ops.cpu_die() on the target CPUs first. At least some implementations
of smp_ops had issues with this; it caused cpu_kill() to hang on Tegra,
for example. Since smp_send_stop() is only used for shutdown, halt, and
power-off, there is no need to attempt any kind of CPU hotplug here.

In machine_shutdown(), replace the call to smp_send_stop() with a call to
disable_nonboot_cpus(). This completely disables all but one CPU, thus
satisfying the requirement that only a single CPU be active for kexec.
Adjust Kconfig dependencies for this change.

In machine_halt()/power_off()/restart(), call smp_send_stop() directly,
rather than via machine_shutdown(); these functions don't need to
completely de-activate all CPUs using hotplug, but rather just quiesce
them.

In machine_kexec(), error out if multiple CPUs are still online. kexec
uses soft_restart(), which won't work if multiple CPUs are active. This
case can be hit on an SMP system that doesn't support CPU hotplug, where
the call to disable_nonboot_cpus() from machine_shutdown() would have no
effect.

Suggested-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
---
v2:
* Removed some unnecessary #ifdefs.
* Simplified Kconfig depends for kexec.
* Replace BUG()s with kinder error-checking code.
* Significantly structurally reworked.