net/mlx5: support PCI device DMA map and unmap

net/mlx5: support PCI device DMA map and unmap

The implementation reuses the external memory registration work done by
commit[1].

Note about representors:

The current representor design will not work
with those map and unmap functions. The reason is that for representors
we have multiple IB devices share the same PCI function, so mapping will
happen only on one of the representors and not all of them.

While it is possible to implement such support, the IB representor
design is going to be changed during DPDK19.05. The new design will have
a single IB device for all representors, hence sharing of a single
memory region between all representors will be possible.

[1]
commit 7e43a32ee060
("net/mlx5: support externally allocated static memory")

Signed-off-by: Shahaf Shuler <shahafs@mellanox.com>