Perf: arm64: support hotplug and power collapse

Add notifiers for hotplug and power collapse in perf events.
Preserve counters and user enables across these events.
Disable and enable interrupts across events.
This allows perf-events to be used in an environment where
cores are power collapsing and being hotplugged.

Change-Id: Id27ac3b1c0733677e0bc9740ce7534c3e5fdac7c
Signed-off-by: Neil Leeder <nleeder@codeaurora.org>