net: dsa: mv88e6xxx: export g2 register accessors

Let the mv88e6xxx_g2_* register accessor functions be accessible
outside of global2.c.

Signed-off-by: Brandon Streiff <brandon.streiff@ni.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>