net/mlx5: Add HV VHCA control agent

Control agent is responsible over of the control block (ID 0). It should
update the PF via this block about every capability change. In addition,
upon block 0 invalidate, it should activate all other supported agents
with data requests from the PF.

Upon agent create/destroy, the invalidate callback of the control agent
is being called in order to update the PF driver about this change.

The control agent is an integral part of HV VHCA and will be created
and destroy as part of the HV VHCA init/cleanup flow.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>