net/mlx5: Add Representors registration API

Introduce E-Switch registration/unregister representors functions.

Those functions are called by the mlx5e driver when the PF NIC is
created upon pci probe action regardless of the E-Switch mode (NONE,
LEGACY or OFFLOADS).

Adding basic E-Switch database that will hold the vport represntors
upon creation.

This patch doesn't add any new functionality.

Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>