perf: enable perf to continue across hotplug

Currently perf hardware, software and tracepoint events are
deleted when a cpu is hotplugged out. This change restarts
the events after hotplug. In arm_pmu.c most of the code
for handline power collapse is reused for hotplug.
This change supercedes commit 1f0f95c5fe9e ("perf: add hotplug
support so that perf continues after hotplug") and uses the
new hotplug notification method.

Change-Id: Ia9de1c150558125e2da5f2c4354c268ce4b1154a
Signed-off-by: Patrick Fay <pfay@codeaurora.org>