net/mlx5: add memory region callbacks in per-device cache

net/mlx5: add memory region callbacks in per-device cache

Prior to this commit MR operations were verbs based and hard coded under
common/mlx5/linux directory. This commit enables upper layers (e.g.
net/mlx5) to determine which MR operations to use. For example the net
layer could set devx based MR operations in non-Linux environments. The
reg_mr and dereg_mr callbacks are added to the global per-device MR
cache 'struct mlx5_mr_share_cache'.

Signed-off-by: Ophir Munk <ophirmu@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>