net: dsa: mv88e6xxx: extract device mapping

The Device Mapping register is an indirect table access.

Provide helpers to access this table and explicit the checking of the
new DSA_RTABLE_NONE routing table value.

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>