perf: add hotplug support

The change is a squash of the following two commits:

1) 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.

2) disable perf_event_read during hotplug:
core.c should not allow perf_event_read access during hotplug.
DCVS may try to read events during hotplug startup or
shutdown. Set a flag to not allow access during hotplug.

Change-Id: I3c5f1f532d451a096d2d3ee976e0a15fca826e8b
Signed-off-by: Patrick Fay <pfay@codeaurora.org>
[rananta@codeaurora.org: resolved trivial conflicts]
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>