Perf: keep events across hotplug

Keep event list alive across a CPU hotplug so that perf
can resume when the CPU comes back online. Bring a CPU
online when exiting a perf session so it can be cleaned
up properly.

Change-Id: Ie0e4a43f751beb77afdc84e9d52b21780f279d80
Signed-off-by: Neil Leeder <nleeder@codeaurora.org>
Signed-off-by: Mekala Natarajan <mekalan@codeaurora.org>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>
Signed-off-by: Chet Kener <Cl3Kener@gmail.com>

Conflicts:
	arch/arm/kernel/perf_event.c
	arch/arm/mach-msm/perf_debug.c
	kernel/events/core.c