perf: llcc: Preserve events across CPU hotplugs

By default, the perf core detaches the event from the PMU when it's
associated CPU hotplugs out. Set flag "events_across_hotplug" to
preserve the events created for LLCC PMU to be preserved across
hotplugs.

Change-Id: Ia4bd4962370dd18fcd4b221905186faadc618ce0
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>