net: add ethernet driver for MediaTek MT7620 SoC

This patch adds  ethernet driver for MediaTek MT7620 SoC.

The MT7620 SoC has a built-in ethernet (Frame Engine) and a built-in
7-port switch and two xMII interfaces (can be MII/RMII/RGMII).

The port 0-3 of the switch connects to intergrited FE PHYs. Port 4 can be
configured to connect to either the intergrited FE PHY, or the xMII.
Port 5 always connects to the xMII. Port 6 is the CPU port.

This driver supports MT7530 giga switch connects to port 5.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>