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: Ib85f948022c029bb5c408e8ae4759077fd7ab0e5
Signed-off-by: Neil Leeder <nleeder@codeaurora.org>