net: dsa: mv88e6xxx: use helper for 6390 histogram

The Marvell 88E6390 model has its histogram mode bits moved in the
Global 1 Control 2 register. Use the previously introduced
mv88e6xxx_g1_ctl2_mask helper to set them.

At the same time complete the documentation of the said register.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>