net/mlx5: Update dead links in Kconfig documentation

Point to NVIDIA documentation for device specific information now that the
Mellanox documentation site is deprecated. Refer to kernel documentation
sources for generic information not specific to mlx5 devices.

Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>