net: dsa: mv88e6xxx: add RXNFC support

Implement the .get_rxnfc and .set_rxnfc DSA operations to configure
a port's Layer 2 Policy Control List (PCL) via ethtool.

Currently only dropping frames based on MAC Destination or Source
Address (including the option VLAN parameter) is supported.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>