net: dsa: mv88e6xxx: Link aggregation support

Support offloading of LAGs to hardware. LAGs may be attached to a
bridge in which case VLANs, multicast groups, etc. are also offloaded
as usual.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>