ramips: add support for TP-Link TL-MR3020 v3

TP-Link TL-MR3020 v3 is a pocket-size router based on MediaTek MT7628N.

This PR is based on the work of @meyergru[1], with his permission.

Specification:
- MediaTek MT7628N/N (575 Mhz)
- 64 MB of RAM
- 8 MB of FLASH
- 2T2R 2.4 GHz
- 1x 10/100 Mbps Ethernet

Flash instruction:

The only way to flash the image in TL-MR3020 v3 is to use
tftp recovery mode in U-Boot:

1. Configure PC with static IP 192.168.0.225/24 and tftp server.
2. Rename "openwrt-ramips-mt76x8-tplink_tl-mr3020-v3-squashfs-tftp-recovery.bin"
   to "tp_recovery.bin" and place it in tftp server directory.
3. Connect PC with the LAN port, press the reset button, power up
   the router and keep button pressed for around 6-7 seconds, until
   device starts downloading the file.
4. Router will download file from server, write it to flash and reboot.

[1] https://github.com/meyergru/lede-source/commits/TL-MR3020-V3

Signed-off-by: Carlo Nel <carlojnel@gmail.com>

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

@@ -404,6 +404,11 @@ tplink,tl-wa801nd-v5)
 	ucidef_set_led_wlan "wlan" "wlan" "$boardname:green:wlan" "phy0tpt"
 	ucidef_set_led_netdev "lan" "lan" "$boardname:green:lan" "eth0"
 	;;
+tplink,tl-mr3020-v3)
+	set_usb_led "$boardname:green:3g"
+	set_wifi_led "$boardname:green:wlan"
+	ucidef_set_led_netdev "lan" "LAN" "$boardname:green:lan" "eth0"
+	;;
 tplink,tl-mr3420-v5|\
 tplink,tl-wr842n-v5)
 	set_usb_led "$boardname:green:usb"

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

@@ -150,6 +150,7 @@ ramips_setup_interfaces()
 	pbr-d1|\
 	ravpower,wd03|\
 	tama,w06|\
+	tplink,tl-mr3020-v3|\
 	u25awf-h1|\
 	wli-tx4-ag300n|\
 	wmdr-143n|\