cpufreq: Protect against hotplug in cpufreq_register_driver()

cpufreq_register_driver() could race with CPU hotplug during
bootup. Since hotplug notification is not registered when
subsys_interface_register() is being executed, it's possible
cpufreq's view of online CPUs becomes stale before it registers
for hotplug notification.

Register for hotplug notification first and protect
subsys_interface_register() against hotplug using
get/put_online_cpus().

Change-Id: I26b2908f1d167c2becc4e8664c357bb7c6162406
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>