ramips: add support for SNR-CPE-ME1

ramips: add support for SNR-CPE-ME1

SNR-CPE-ME1 is a wireless WiFi 5 router manufactured by SNR/NAG company.

Specification:
    - SoC           : MediaTek MT7621A
    - RAM           : DDR3 256 MiB
    - Flash         : SPI-NOR 16 MiB (GD25Q128CSIG)
    - WLAN          : 2.4 GHz (MediaTek MT7603EN)
                      5 GHz (MediaTek MT7610EN)
    - Ethernet      : 10/100/1000 Mbps x5
      - Switch      : MediaTek MT7530 (in SoC)
    - USB           : 3.0 x1
    - UART          : through-hole on PCB
      - [J4] 3.3V, RX, TX, GND (57600n8)
    - Power         : 12 VDC, 2 A

Flash instruction via TFTP:
    1. Boot SNR-CPE-ME1 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
    4. Do sysupgrade using web-interface

Signed-off-by: Maximilian Weinmann <x1@disroot.org>