perf/x86: Detect number of instances of uncore CBox

The CBox manages the interface between the core and the LLC, so
the instances of uncore CBox is equal to number of cores.

Reported-by: Andrew Cooks <acooks@gmail.com>
Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1341381616-12229-4-git-send-email-zheng.z.yan@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>