perf events, x86: Add SandyBridge stalled-cycles-frontend/backend events

Extend the Intel SandyBridge PMU driver with definitions
for generic front-end and back-end stall events.

( As commit 30112039 "perf events, x86: Add Westmere stalled-cycles-frontend/backend
  events" says, these are only approximations. )

Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1304666042-17577-1-git-send-email-ming.m.lin@intel.com


Signed-off-by: Ingo Molnar <mingo@elte.hu>