powerpc: Move arch_cpu_idle_dead() into smp.c

arch_cpu_idle_dead() is in idle.c, which makes sense, but it's inside
a CONFIG_HOTPLUG_CPU block.

It would be more at home in smp.c, inside the existing
CONFIG_HOTPLUG_CPU block. Note that CONFIG_HOTPLUG_CPU depends on
CONFIG_SMP so even though smp.c is not built for SMP=n builds, that's
fine.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200819015634.1974478-1-mpe@ellerman.id.au

arch/powerpc/kernel/idle.c

@@ -41,14 +41,6 @@ static int __init powersave_off(char *arg)
 }
 __setup("powersave=off", powersave_off);
 
-#ifdef CONFIG_HOTPLUG_CPU
-void arch_cpu_idle_dead(void)
-{
-	sched_preempt_enable_no_resched();
-	cpu_die();
-}
-#endif
-
 void arch_cpu_idle(void)
 {
 	ppc64_runlatch_off();

arch/powerpc/kernel/smp.c

@@ -1489,6 +1489,12 @@ void __cpu_die(unsigned int cpu)
 		smp_ops->cpu_die(cpu);
 }
 
+void arch_cpu_idle_dead(void)
+{
+	sched_preempt_enable_no_resched();
+	cpu_die();
+}
+
 void cpu_die(void)
 {
 	/*