net: dsa: b53: Provide number of ARL buckets

In preparation for doing proper upper bound checking of FDB/MDB entries
being added to the ARL, provide the number of ARL buckets for each
switch chip we support. All chips have 1024 buckets, except 7278 which
has only 256.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>