mlx4: new poll mode driver

mlx4: new poll mode driver

This PMD manages all variants of Mellanox ConnectX-3 (EN 40, EN 10, Pro EN
40) as well as their virtual functions in SR-IOV context through IB Verbs
(libibverbs) and the dedicated user-space driver (libmlx4).

It is disabled by default due to dependencies on these libraries and only
supports Linux userland at the moment partly because /sys (sysfs) support is
required.

Also claim responsibility in the MAINTAINERS file.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Signed-off-by: Olga Shern <olgas@mellanox.com>