net: mscc: ocelot: move stats code to ocelot_stats.c

The main C file of the ocelot switch lib, ocelot.c, is getting larger
and larger, and there are plans to add more logic related to stats.
So it seems like an appropriate moment to split the statistics code to a
new file.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>