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>

target/linux/ramips/mt7620/base-files/etc/board.d/01_leds

@@ -151,6 +151,11 @@ kingston,mlwg2|\
 sanlinking,d240)
 	ucidef_set_led_netdev "wifi_led" "wifi" "blue:wifi" "wlan0"
 	;;
+snr,cpe-w4n-mt)
+	ucidef_set_led_heartbeat "wps" "WPS" "green:wps"
+	ucidef_set_led_timer "sys" "System" "green:sys" "500" "500"
+	ucidef_set_led_wlan "wlan" "Wi-Fi" "green:wlan" "phy0tpt"
+	;;
 lenovo,newifi-y1)
 	ucidef_set_led_netdev "wifi" "WIFI" "blue:wifi" "wlan1"
 	ucidef_set_led_netdev "wifi5g" "WIFI5G" "blue:wifi5g" "wlan0"

target/linux/ramips/mt7620/base-files/etc/board.d/02_network

@@ -86,6 +86,7 @@ ramips_setup_interfaces()
 	glinet,gl-mt300n|\
 	glinet,gl-mt750|\
 	hiwifi,hc5661|\
+	snr,cpe-w4n-mt|\
 	wrtnode,wrtnode|\
 	zbtlink,zbt-wa05|\
 	zyxel,keenetic-omni-ii)
@@ -381,6 +382,11 @@ ramips_setup_macs()
 	linksys,e1700)
 		wan_mac=$(mtd_get_mac_ascii config WAN_MAC_ADDR)
 		;;
+	snr,cpe-w4n-mt)
+		lan_mac=$(mtd_get_mac_binary Factory 0x28)
+		wan_mac=$(mtd_get_mac_binary Factory 0x2e)
+		label_mac=$wan_mac
+		;;
 	tplink,archer-c2-v1|\
 	tplink,archer-c20-v1|\
 	tplink,archer-c20i|\