ramips: add support for SNR-CPE-W4N-MT router

ramips: add support for SNR-CPE-W4N-MT router

General specification:
- SoC Type: MediaTek MT7620N (580MHz)
- ROM: 8 MB SPI-NOR (W25Q64FV)
- RAM: 64 MB DDR (M13S5121632A)
- Switch: MediaTek MT7530
- Ethernet: 5 ports - 5×100MbE (WAN, LAN1-4)
- Wireless 2.4 GHz: b/g/n
- Buttons: 1 button (RESET)
- Bootloader: U-Boot 1.1.3, MediaTek U-Boot: 5.0.0.5
- Power: 12 VDC, 1.0 A

Flash by the native uploader in 2 stages:
1. Use the native uploader to flash an initramfs image. Choose
 openwrt-ramips-mt7620-snr_cpe-w4n-mt-initramfs-kernel.bin file by
 "Administration/Management/Firmware update/Choose File" in vendor's
 web interface (ip: 192.168.1.10, login: Admin, password: Admin).
 Wait ~160 seconds.
2. Flash a sysupgrade image via the initramfs image. Choose
 openwrt-ramips-mt7620-snr_cpe-w4n-mt-squashfs-sysupgrade.bin
 file by "System/Backup/Flash Firmware/Flash image..." in
 LuCI web interface (ip: 192.168.1.1, login: root, no password).
 Wait ~240 seconds.

Flash by U-Boot TFTP method:
1. Configure your PC with IP 192.168.1.131
2. Set up TFTP server and put the
 openwrt-ramips-mt7620-snr_cpe-w4n-mt-squashfs-sysupgrade.bin
 image on your PC
3. Connect serial port (57600 8N1) and turn on the router.
 Then interrupt "U-Boot Boot Menu" by hitting 2 key (select "2:
 Load system code then write to Flash via TFTP.").
Press Y key when show "Warning!! Erase Linux in Flash then burn
 new one. Are you sure? (Y/N)"
Input device IP (192.168.1.1) ==:192.168.1.1
Input server IP (192.168.1.131) ==:192.168.1.131
Input Linux Kernel filename () ==:
openwrt-ramips-mt7620-snr_cpe-w4n-mt-squashfs-sysupgrade.bin
3. Wait ~120 seconds to complete flashing

Signed-off-by: Alexey Bartenev <41exey@proton.me>