dt-bindings: net: mediatek: Add support for MediaTek MT7628/88 SoC

Add compatible for the ethernet IP core on MT7628/88 SoCs. Its
compatible with the older Ralink Rt5350F SoC. And OpenWrt already
uses this compatible string for the MT76x8.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: René van Dorst <opensource@vdorst.com>
Cc: Daniel Golle <daniel@makrotopia.org>
Cc: Sean Wang <sean.wang@mediatek.com>
Cc: John Crispin <john@phrozen.org>
Cc: devicetree@vger.kernel.org
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>