perf/x86/intel/uncore: Add box_offsets for free-running counters

The offset between uncore boxes of free-running counters varies, e.g.
IIO free-running counters on Ice Lake server.

Add box_offsets, an array of offsets between adjacent uncore boxes.

Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/1584470314-46657-1-git-send-email-kan.liang@linux.intel.com