octeontx2-af: Add CGX LMAC stats to debugfs

This patch adds CGX LMAC physical interface or serdes Rx/Tx
packet stats to debugfs.

'cat cgx<idx>/lmac<idx>/stats' dumps the current interface link
status and Rx/Tx stats. Stats include pkt received/transmitted,
dropped, pause frames etc etc.

Signed-off-by: Prakash Brahmajyosyula <bprakash@marvell.com>
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>