octeontx2-af: Support to retrieve CGX LMAC stats

This patch adds support for a RVU PF/VF driver to retrieve
it's mapped CGX LMAC Rx and Tx stats from AF via mbox.
New mailbox msg is added is added.

Signed-off-by: Christina Jacob <cjacob@marvell.com>
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>