octeontx2-af: Add per CGX port level NIX Rx/Tx counters

A CGX port is shared by a RVU PF and it's VFs. These per
CGX port level NIX Rx/Tx counters are cumilative stats of
all NIXLFs sharing this port. These stats when compared
to CGX Rx/Tx stats helps in identifying pkts dropped within
the system, if any.

Signed-off-by: Linu Cherian <lcherian@marvell.com>
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>