powerpc/powermac: Convert to hotplug state machine

Install the callbacks via the state machine.
I assume here that the powermac has two CPUs and so only one can go up
or down at a time. The variable smp_core99_host_open is here to ensure
that we do not try to open or close the i2c host twice if something goes
wrong and we invoke the prepare or online callback twice due to
rollback.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: rt@linutronix.de
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: linuxppc-dev@lists.ozlabs.org
Link: http://lkml.kernel.org/r/20160818125731.27256-16-bigeasy@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/cpuhotplug.h

@@ -31,6 +31,7 @@ enum cpuhp_state {
 	CPUHP_MD_RAID5_PREPARE,
 	CPUHP_RCUTREE_PREP,
 	CPUHP_CPUIDLE_COUPLED_PREPARE,
+	CPUHP_POWERPC_PMAC_PREPARE,
 	CPUHP_NOTIFY_PREPARE,
 	CPUHP_TIMERS_DEAD,
 	CPUHP_BRINGUP_CPU,