net: mscc: ocelot: debugging print for statistics regions

To make it easier to debug future issues with statistics counters not
getting aggregated properly into regions, like what happened in commit
6acc72a4

 ("net: mscc: ocelot: fix stats region batching"), add some
dev_dbg() prints which show the regions that were dynamically
determined.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>