net: dsa: mv88e6xxx: move VTU Operation accessors

Move the helper functions to access the Global 1 VTU Operation register
to a new global1_vtu.c file, and get rid of the old underscore prefix
naming convention. This file will be extended will all VTU/STU related
code.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>