devlink: add documentation for octeontx2 driver

Add a file to document devlink support for octeontx2
driver. Driver-specific parameters implemented by
AF, PF and VF drivers are documented.

Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>