net/mlx5: Extend mlx5_core to support ConnectX-4 Ethernet functionality

This is the Ethernet part of the driver for the Mellanox ConnectX(R)-4
Single/Dual-Port Adapter supporting 100Gb/s with VPI.  The driver
extends the existing mlx5 driver with Ethernet functionality.

This patch contains the driver entry points but does not include
transmit and receive (see the previous patch in the series) routines.

It also adds the option MLX5_CORE_EN to Kconfig to enable/disable the
Ethernet functionality. Currently, Kconfig is programmed to make
Ethernet and Infiniband functionality mutally exclusive.
Also changed MLX5_INFINIBAND to be depandant on MLX5_CORE instead of
selecting it, since MLX5_CORE could be selected without MLX5_INFINIBAND
being selected.

Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>