net/mlx5: update memory event callback for shared context

net/mlx5: update memory event callback for shared context

Mellanox mlx5 PMD implements the list of devices to process the memory
free events to reflect the actual memory state to Memory Regions.
Because this list contains the devices and devices may share the
same context the callback routine may be called multiple times
with the same parameter, that is not optimal. This patch modifies
the list to contain the device contexts instead of device objects
and shared context is included in the list only once.

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>