net/mlx5: E-Switch, Add callback to get representor device

Add a callback interface to get a protocol device (per representor type).
The Ethernet representors will expose their netdev via this interface.

This functionality can be later used by IB representor in order to find the
corresponding net device representor.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>