ramips: add support for Mercusys MR70X

ramips: add support for Mercusys MR70X

Hardware
========
- SoC: MediaTek MT7621AT (880MHz, Duel-Core)
- RAM: DDR3 128MB
- Flash: Winbond W25Q128JV (SPI-NOR 16MB)
- WiFi: MediaTek MT7915D (2.4GHz, 5GHz, DBDC)
- Ethernet: MediaTek MT7530 (WAN x1, LAN x3, SoC)
- UART: >TX RX GND 3v3 (115200 8N1, J1)
        Do not connect 3v3. TX is marked with an arrow.

Installation
============
Flash factory image. This can be done using stock web ui.

Revert to stock firmware
========================
Flash stock firmware via OEM Web UI Recovery mode.

Web UI Recovery method
======================
1. Unplug the router
2. Plug in and hold reset button 5~10 secs
3. Set your computer IP address manually to 192.168.1.x / 255.255.255.0
4. Flash image with web browser to 192.168.1.1

Co-authored-by: Robert Senderek <robert.senderek@10g.pl>
Co-authored-by: Yoonji Park <koreapyj@dcmys.kr>
Signed-off-by: David Bauer <mail@david-bauer.net>