arm64: smp: Prevent hotplugging the last AArch32-able CPU

EL0 AArch32 tasks are now sigkilled when they can't run on any
compatible CPU, either because there aren't any left (hotplug) or
because they aren't allowed to run on those left (task affinity).

However, it has been deemed valuable to prevent the loss of
functionality resulting in offlining the last AArch32-compatible CPU.

Add arch-specific hook in _cpu_down() that allows checking whether we
can offline a cpu or not and use that hook to veto offlining the last
AArch32 CPU.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
[Qais: Reworked to use a arch hook and moved the code to arch/arm64.
Added handling of offlining a cpu due to suspend.]
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Change-Id: Ibc77e948a118248c7899d97b64194a934ddbf527