common/mlx5: share device context object

common/mlx5: share device context object

Create shared context device in common area and add it as a field of
common device.
Use this context device in all drivers and remove the ctx field from
their private structure.

Signed-off-by: Michael Baum <michaelba@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>