net: dsa: mv88e6xxx: add phylink_get_caps for the mv88e6320/21 family

net: dsa: mv88e6xxx: add phylink_get_caps for the mv88e6320/21 family

As of commit de5c9bf40c45 ("net: phylink: require supported_interfaces to
be filled")
Marvell 88e6320/21 switches fail to be probed:

...
mv88e6085 30be0000.ethernet-1:00: phylink: error: empty supported_interfaces
error creating PHYLINK: -22
...

The problem stems from the use of mv88e6185_phylink_get_caps() to get
the device capabilities.
Since there are serdes only ports 0/1 included, create a new dedicated
phylink_get_caps for the 6320 and 6321 to properly support their
set of capabilities.

Fixes: de5c9bf40c45 ("net: phylink: require supported_interfaces to be filled")
Signed-off-by: Steffen Bätz <steffen@innosonix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20240508072944.54880-2-steffen@innosonix.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>