doc: announce ABI change for link speed

doc: announce ABI change for link speed

A rework was prepared by Marc Sune:
http://dpdk.org/ml/archives/dev/2015-October/026037.html
The goal is to retrieve the supported link speed of a device
and to allow 100G devices while having a consistent API.

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Olga Shern <olgas@mellanox.com>
Acked-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Jan Viktorin <viktorin@rehivetech.com>
Acked-by: Matej Vido <matejvido@gmail.com>