net: Add MT7628 ethernet driver

net: Add MT7628 ethernet driver

This patch adds ethernet support for the MIPS based Mediatek MT76xx SoCs
(e.g. MT7628 and MT7688), including a minimum setup of the integrated
switch. This driver is loosly based on the driver version included in
this MediaTek github repository:

https://github.com/MediaTek-Labs/linkit-smart-uboot.git

Tested on the MT7688 LinkIt smart-gateway and on the
Gardena-smart-gateway.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Frank Wunderlich <frankwu@gmx.de>
Cc: Weijie Gao <hackpascal@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>