ramips: add support for TP-Link MR600 V2(EU)

ramips: add support for TP-Link MR600 V2(EU)

MR600 V2(EU) is an LTE router that also supports 4G+ band aggregation
etc. and can reportedly achieve higher bandwidth with it.

- Specifications:

* SoC: Mediatek MT7621DAT 880MHz
* RAM: 128MB DDR3
* Flash: 16MB SPI NOR flash (GD25Q128C)
* LTE Modem: Qualcomm MDM9240
* WiFi 5GHz: Mediatek MT7613BEN
* WiFi 2.4GHz: Mediatek MT7603EN
* Ethernet: MT7530, 4x 1000Base-T.
* UART: Serial console (115200 8n1), J1(GND:3)
* Buttons: Reset, WPS.
* LED: Power, WAN, LTE, WiFi 2GHz and 5GHz, LAN, Signal1, Signal2,
  Signal3

- MAC Addresses:

OEM firmware configuration:
54:af:97:xx:xx:7b : 2.4G
54:af:97:xx:xx:7a : 5G
54:af:97:xx:xx:7c : LTE
54:af:97:xx:xx:7b : LAN (label)
54:af:97:xx:xx:7c : WAN

- Installation:

1. Download the OpenWrt initramfs-image.

Place it into a TFTP server root directory and rename it to openwrt.img
Configure the TFTP server to listen at 192.168.0.5/24.

3. Connect to the serial console.

Attach power and interrupt the boot procedure when prompted (type `tpl`).

Credentials are admin / 1234

4. Configure U-Boot for booting OpenWrt from ram

 $ tftpboot
 $ bootm

5. Transfer the OpenWrt sysupgrade image to the device.

- LTE:

In order to setup the wwan0 interface:
1. Add a `qmi` proto interface under `/etc/config/network`, e.g.:
```
config interface 'wwan0'
        option device '/dev/cdc-wdm0'
        option proto 'qmi'
        option pincode 'XXXX'
        option apn 'your_isp_apn'
```

2. Add `wwan0` interface to the `wan` firewall zone
3. `/etc/init.d/network restart`

Signed-off-by: Linos Giannopoulos <linosgian00@gmail.com>