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>

doc/device-tree-bindings/net/mediatek,mt7628-eth.txt

@@ -0,0 +1,17 @@
+* MediaTek Frame Engine Ethernet controller
+
+Required properties:
+- compatible: should be "mediatek,mt7628-eth"
+- reg: address and length of the register set for the frame
+       engine ethernet controller and the internal switch.
+- syscon: phandle to the system controller
+
+Example:
+
+eth@10100000 {
+	compatible = "mediatek,mt7628-eth";
+	reg = <0x10100000 0x10000
+	       0x10110000 0x8000>;
+
+	syscon = <&sysc>;
+};