net/mlx5: Implement SRIOV attach/detach flows

Needed for lightweight and modular internal/pci error handling.
Implement sriov attach function which enables pre-saved number of vfs on
the device side.
Implement sriov detach function which disable the current vfs on the
device side.
Init/cleanup function only handles sriov 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>