net/mlx5: Implement eswitch attach/detach flows

Needed for lightweight and modular internal/pci error handling.
Implement eswitch attach function which allocates/starts hw related
resources.
Implement eswitch detach function which releases/stops hw related
resources.
Init/cleanup function only handle eswitch software context allocation
and destruction.

Signed-off-by: Mohamad Haj Yahia <mohamad@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>