[DRIVERS] net: Moving ethernet drivers under "net/ethernet" directory

Following Linux style to maintain ethernet drivers under net/ethernet
directory.

Signed-off-by: Pranav Sawargaonkar <pranav.sawargaonkar@gmail.com>
[ARM] Update all defconfigs for networking options.

My recent patch has changed the networking directory layout and
menuconfig options.

Syncing all the defconfigs which used to use "CONFIG_NET_DRIVERS" to
use "CONFIG_NET_DEVICES" and "CONFIG_ETHERNET_DRIVERS".

Signed-off-by: Pranav Sawargaonkar <pranav.sawargaonkar@gmail.com>