ramips: mt76x8: add support for Teltonika RUT200

ramips: mt76x8: add support for Teltonika RUT200

Specification:
SoC: MediaTek MT7628AN
RAM: 128 MB EtronTech EM68C16CWQG-25IH
Flash: 16MB Winbond W25Q128 SPI
Switch: MediaTek MT7628AN, 2 ports 100 Mbps
WiFi: MediaTek MT7628AN 2.4 GHz 802.11n
Modem: Quectel EC200A 4G, cat 4
GPIO:
  - 1 button (Reset)
  - 8 LEDs (2G, 3G, 4G, RSSI 1,2,3,4,5)
  - 2 Modem control (power button, reset)
  - 1 Digital input
  - 1 Digital output

Flashing via OEM WebUI:
1. Download the firmware image *-squashfs-factory.bin
2. Upload firmware image via OEM WebUI firmware update, do not keep settings

To revert back to OEM firmware:
https://wiki.teltonika-networks.com/view/Bootloader_menu

To enable mobile data connection send command to modem:
  echo -ne 'AT+QNETDEVCTL=3,1\r\n' > /dev/ttyUSB2

Create DHCP interface with usb0 device.

Signed-off-by: Simonas Tamošaitis <simsasss@gmail.com>