net: dsa: mv88e6xxx: Add stats_get_sset_count|string to ops structure

Different families have different sets of statistics. Abstract this
using a stats_get_sset_count and stats_get_strings op. Each stat has a
bitmap, and the ops implementer uses a bit map mask to count the
statistics which apply for the family, or return the list of strings.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
v2:
  Rename functions to avoid _ prefix.
Signed-off-by: David S. Miller <davem@davemloft.net>