net/mlx5e: Add neighbour hash table to the representors

Add hash table to the representors which is to be used by the next patch
to save neighbours information in the driver.

In order to offload IP tunnel encapsulation rules, the driver must find
the tunnel dst neighbour according to the output device and the
destination address given by the user. The next patch will cache the
neighbors information in the driver to allow support in neigh update
flow for tunnel encap rules.

The neighbour entries are also saved in a list so we easily iterate over
them when querying statistics in order to provide 'used' feedback to the
kernel neighbour NUD core.

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