net/mlx5e: IPoIB, Add netdevice profile skeleton

Create mlx5e IPoIB netdevice profile skeleton in the new ipoib.c
file with empty implementation.

Downstream patches will provide the full mlx5 rdma netdevice acceleration
support for IPoIB into this new file, by using the mlx5e netdevice
profile and new mlx5_channels APIs and infrastructures.
Same as already done in mlx5e NIC netdevice and switchdev mode VF
representors.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Erez Shitrit <erezsh@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>