perf/x86: Support CPU specific sysfs events

Add a way for the CPU initialization code to register additional
events, and merge them into the events attribute directory. Used
in the next patch.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Stephane Eranian <eranian@google.com>
Cc: peterz@infradead.org
Cc: acme@redhat.com
Cc: jolsa@redhat.com
Cc: namhyung.kim@lge.com
Link: http://lkml.kernel.org/r/1359040242-8269-2-git-send-email-eranian@google.com
[ small cleanups ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
[ merge_attr returns a **, not just * ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>