ramips: add Edimax BR-6208AC V2 support

Specifications:
- Device: Edimax BR-6208AC V2
- SoC: MT7620A
- Flash: 16 MiB
- RAM: 64 MiB
- Switch: 1 WAN, 3 LAN (10/100 Mbps)
- WiFi: MT7620 2.4 GHz + MT7610E 5 GHz
- LEDs: 1x POWER (green, not configurable)
        1x Firmware (green, configurable)
        1x Internet (green, configurable)
        1x VPN (green, configurable)
        1x 2.4G (green, not configurable)
        1x 5G (green, not configurable)

Normal installation:
- Upload the sysupgrade image via the default web interface

Installation with U-Boot and TFTP:
- Requires a TFTP server which provides the sysupgrade image
- Requires a connection to the serial port of the device, rate 57600

Signed-off-by: Stefan Weil <sw@weilnetz.de>
(cherry picked from commit 8d06bc175154db42a0550212a4b8474008939b34)