cpufreq: Register for hotplug notifier before locking hotplug

If a CPU hotplug operation occurs in parallel while a driver is
registering with the cpufreq framework it can lead to the following
deadlock scenario:
Thread A (cpufreq_register_driver())	Thread B (cpu_down())
get_online_cpus()
	|
atomic_inc(&cpu_hotplug.refcount)
					     cpu_down()
						|
				     mutex_lock(&cpu_add_remove_lock)
						|
				     cpu_hotplug_begin() waits on
				     cpu_hotplug.refcount to reset
register_cpu_notifier()
	|
mutex_lock(&cpu_add_remove_lock)

This happens because the registration for hotplug notifiers happens
inside the hotplug locked section.
This change moves hotplug registration call before get_online_cpus()
to avoid the deadlock.

Change-Id: Ia59a3e2710133d3b3608e49938e746e58af5a6d4
Signed-off-by: Rohit Gupta <rohgup@codeaurora.org>