net: mvpp2: enable UDP/TCP checksum over IPv6

This patch adds the NETIF_F_IPV6_CSUM to the driver's features to enable
UDP/TCP checksum over IPv6. No extra configuration of the engine is
needed on top of the IPv4 counterpart, which already is in the features
list (NETIF_F_IP_CSUM).

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>