net: mvpp2: cls: Allow dropping packets with classification offload

This commit introduces support for the "Drop" action in classification
offload. This corresponds to the "-1" action with ethtool -N.

This is achieved using the color marking actions available in the C2
engine, which associate a color to a packet. These colors can be either
Green, Yellow or Red, Red meaning that the packet should be dropped.

Green and Yellow colors are interpreted by the Policer, which isn't
supported yet.

This method of dropping using the Classifier is different than the
already existing early-drop features, such as VLAN filtering and MAC
UC/MC filtering, which are performed during the Parsing step, and
therefore take precedence over classification actions.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>