x86/apb_timer: Convert to hotplug state machine

Install the callbacks via the state machine. There is no setup just one
teardown callback. Remove the silly comment about the workqueue up dependency.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: rt@linutronix.de
Link: http://lkml.kernel.org/r/20160713153335.625342983@linutronix.de


Signed-off-by: Ingo Molnar <mingo@kernel.org>