ramips: add support for TOTOLINK LR1200

ramips: add support for TOTOLINK LR1200

Specifications:
- SoC: MT7628DAN (MT7628AN with 64MB built-in RAM)
- Flash: 8M SPI NOR
- Ethernet: 5x 10/100Mbps
- WiFi: 2.4G: MT7628 built-in
        5G: MT7612E
- 1x miniPCIe slot for LTE modem (only USB pins connected)
- 1x SIM slot

Flash instruction:
U-boot has a builtin web recovery page:
1. Hold the reset button while powering it up
2. Connect to the ethernet and set an IP in 192.168.1.0/24 range
3. Open your browser and upload firmware through http://192.168.1.1

Note about the LTE modem:
If your router comes with an EC25 module and it doesn't show up
as a QMI device, you should do the following to switch it to QMI
mode:
1. Install kmod-usb-serial-option and a terminal software
   (e.g. minicom or screen). All 4 serial ports of the modem
   should be available now.
2. Open /dev/ttyUSB3 with the terminal software and type this
   AT command: AT+QCFG="usbnet",0
3. Power-cycle the router. You should now get a QMI device
   recognized.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>