net: dsa: mv88e6xxx: Implement mv88e6390 tag remap

The mv88e6390 does not have the two registers to set the frame
priority map. Instead it has an indirection registers for setting a
number of different priority maps. Refactor the old code into an
function, implement the mv88e6390 version, and use an op to call the
right one.

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