net: dsa: mv88e6xxx: add VTU Load/Purge operation

Add a new vtu_loadpurge operation to the chip info structure to differ
the various implementations of the VTU accesses.

Now that the STU handling is abstracted behind VTU operations, kill the
obsolete MV88E6XXX_FLAG_STU flag.

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>