net: dsa: mv88e6xxx: Add number of MACs in the ATU

For each supported switch, add an entry to the info structure for the
number of MACs which can be stored in the ATU. This will later be used
to export the ATU as a devlink resource, and indicate its occupancy,
how full the ATU is.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>