Perf: arm: 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>