perf c2c report: Add total loads sort key

It is to be displayed in the main cachelines overall output:

  tot_loads

It displays sum of all load accesses for cacheline.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Joe Mario <jmario@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-czd17qsh5u5z0yc1estz9l2y@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>