net/mlx5e: Extendable vport representor netdev private data

Make representor netdev private data extendable by adding new struct
"mlx5e_rep_priv" and use it as the rep netdev private data struct
instead of directly pointing to mlx5_eswitch_rep.

Added new en_rep.h header file to contain all representor related
definitions and prototypes, and moved all representor specific logic
into en_rep.c.

Needed for downstream patches to extend representor functionality to
support neighbour update.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>