net: sparx5: add support for mrouter ports

All multicast should be forwarded to mrouter ports. Mrouter ports must
therefore be part of all active multicast groups, and override flooding
from being disabled.

Signed-off-by: Casper Andersson <casper.casan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>