net: dsa: mv88e6xxx: Add mv88e6390 statistics unit init

The statistics unit on the mv88e6390 needs the histogram mode to be
configured in a different register compared to other devices. Add an
ops to do this.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
v2:
  Rename to mv88e6390_g1_stats_set_histogram
  Move into global1.c
Signed-off-by: David S. Miller <davem@davemloft.net>