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

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>
(backported from 6bbb2202551be394fead2efd99eb946f846fc63d)