net: dsa: mv88e6xxx: Mark chips with undocumented EDSA tag support

All devices are capable of using regular DSA tags. Support for
Ethertyped DSA tags sort into three categories:

1. No support. Older chips fall into this category.

2. Full support. Datasheet explicitly supports configuring the CPU
   port to receive FORWARDs with a DSA tag.

3. Undocumented support. Datasheet lists the configuration from
   category 2 as "reserved for future use", but does empirically
   behave like a category 2 device.

So, instead of listing the one true protocol that should be used by a
particular chip, specify the level of support for EDSA (support for
regular DSA is implicit on all chips). As before, we use EDSA for all
chips that fully supports it.

In upcoming changes, we will use this information to support
dynamically changing the tag protocol.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>