ramips: add support for SNR SNR-CPE-ME2-Lite

ramips: add support for SNR SNR-CPE-ME2-Lite

SNR SNR-CPE-ME2-Lite is a 2.4/5 GHz band 11ac (Wi-Fi 5) router, based on MT7621A.

Specification:
    - SoC           : MediaTek MT7621A
    - RAM           : DDR3 128 MiB
    - Flash         : SPI-NOR 16 MiB
    - WLAN          : 2.4 GHz 2T2R (MediaTek MT7603E)
                      5 GHz 2T2R (MediaTek MT7613BE)
    - Ethernet      : 10/100/1000 Mbps x5
      - Switch      : MediaTek MT7530 (SoC)
    - UART          : through-hole on PCB
      - J4: 3.3V, GND, TX, RX
      - 57600n8
    - Power         : 12 VDC, 1.5 A

Flash instruction via WEB (firmware 1.10.4 and later)
    1. Boot SNR-CPE-ME2-Lite normally with "Router" mode
    2. Access to "http://192.168.1.1/" and open "Administration -> Management" page
    3. Select the OpenWrt factory image in "Firmware update" section and click "Update" button
    4. Wait ~120 seconds to complete flashing

Flash instruction via TFTP (all version):
    1. Boot SNR-CPE-ME2-Lite to recovery mode (hold the reset button while power on)
    2. Send firmware via TFTP client:
       TFTP Server address: 192.168.1.1
       TFTP Client address: 192.168.1.131
    3. Wait ~120 seconds to complete flashing

Signed-off-by: Alexey Kosmakov <a.kosmakov@nagtech.ru>