leds/trigger/cpu: Convert to hotplug state machine

This is a straightforward conversion. We place this callback last
in the list so that the LED illuminates only after a successful
bring up sequence.

( NOTE: The patch adds a FIXME question about the callback used,
        this question should probably be revisited later on.)

Signed-off-by: Richard Cochran <rcochran@linutronix.de>
Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-leds@vger.kernel.org
Cc: rt@linutronix.de
Link: http://lkml.kernel.org/r/20160713153336.465496902@linutronix.de


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