ethernet: MediaTek: add ethernet driver for MediaTek ARM-based SoCs

This patch adds ethernet support for Mediatek ARM-based SoCs, including
a minimum setup of the integrated switch.

Cc: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Mark Lee <Mark-MC.Lee@mediatek.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>
Tested-By: "Frank Wunderlich" <frank-w@public-files.de>