net: dsa: sja1105: configure the multicast policers, if present

The SJA1110 policer array is similar in layout with SJA1105, except it
contains one multicast policer per port at the end.

Detect the presence of multicast policers based on the maximum number of
supported L2 Policing Table entries, and make those policers have a
shared index equal to the port's default policer. Letting the user
configure these policers is not supported at the moment.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>