doc: update mlx4 documentation

doc: update mlx4 documentation

This updates mlx4 documentation and DPDK release notes
to reflect the PMD support for rdma-core from linux-rdma.

- PMD is now freed from Mellanox OFED and now only depends on the
  public rdma-core package (v15 and above) instead.
  (see https://github.com/linux-rdma/rdma-core/releases)
  This PMD should run under Linux v4.14 and above.
- In case any of the above requirements can't be satisfied,
  Mellanox OFED v4.2 and above also provide an updated rdma-core
  as well back-ported kernel modules for most Linux distributions
  and previous Linux versions.
  (see http://www.mellanox.com/page/products_dyn?product_family=26).

Signed-off-by: Moti Haimovsky <motih@mellanox.com>
Acked-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>