mlx4: Register mlx4 devices to an auxiliary virtual bus

Add an auxiliary virtual bus to model the mlx4 driver structure. The
code is added along the current custom device management logic.
Subsequent patches switch mlx4_en and mlx4_ib to the auxiliary bus and
the old interface is then removed.

Structure mlx4_priv gains a new adev dynamic array to keep track of its
auxiliary devices. Access to the array is protected by the global
mlx4_intf mutex.

Functions mlx4_register_device() and mlx4_unregister_device() are
updated to expose auxiliary devices on the bus in order to load mlx4_en
and/or mlx4_ib. Functions mlx4_register_auxiliary_driver() and
mlx4_unregister_auxiliary_driver() are added to substitute
mlx4_register_interface() and mlx4_unregister_interface(), respectively.
Function mlx4_do_bond() is adjusted to walk over the adev array and
re-adds a specific auxiliary device if its driver sets the
MLX4_INTFF_BONDING flag.

Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>
Tested-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Acked-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>