octeontx2-af: Enable or disable CGX internal loopback

Add support to enable or disable internal loopback mode in CGX.
New mbox IDs CGX_INTLBK_ENABLE/DISABLE added for this.

Signed-off-by: Geetha sowjanya <gakula@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>