perf tools: Add support to specify pmu style event

Added new event rule to the event definition grammar:

event_def: event_pmu |
           ...
event_pmu: PE_NAME '/' event_config '/'

Using this rule, event could be now specified like:
  cpu/config=1,config1=2,config2=3/u

where pmu name 'cpu' is looked up via following path:
  ${sysfs_mount}/bus/event_source/devices/${pmu}

and config options are bound to the pmu's format definiton:
  ${sysfs_mount}/bus/event_source/devices/${pmu}/format

The hardcoded config options still stays and have precedence
over any format field defined with same name.

Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/n/tip-50d8nr94f8k4wkezutrxvthe@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>