net: dsa: mv88e6xxx: Add ATU occupancy via devlink resources

The ATU can report how many entries it contains. It does this per bin,
there being 4 bins in total. Export the ATU as a devlink resource, and
provide a method the needed callback to get the resource occupancy.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>