net: mvpp2: Add support for more ethtool counters

Besides the MIB counters, some other useful counters can be exposed to
the user. This commit adds support for :

 - Per-port counters, that indicate FIFO drops and classifier drops,
 - Per-rxq counters,
 - Per-txq counters

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