ramips: add support for TP-Link RE210 v1

ramips: add support for TP-Link RE210 v1

Hardware
--------

SoC:   MediaTek MT7620A
RAM:   64MB
FLASH: 8MB SPI
WLAN:  2G: MediaTek MT7620A
       5G: MediaTek MT7610EN
ETH:   1x 10/100/1000M (Atheros AR8035)
LED:   RSSI (orange/green)
       WiFi 2G (green)
       WiFi 5G (green)
       Power (green)
       System (red / green)
BTN:   Power
       Reset
       LED
       WPS

Serial
------

P1 - Tx
P2 - Rx
P3 - GND
P4 - VCC

Pin 4 is the one closest to the LAN port.

MAC overview
------------

WAN *:4c uboot 0x1fc00
2.4 *:4c uboot 0x1fc00
5   *:4e uboot 0x1fc00 +2

Installation
------------

Web interface:
It is possible to upgrade to OpenWrt via the web interface. However, the
OEM firmware upgrade file is required and a tool to fix the MD5 sum of
the header. This procedure overwrites U-Boot and there is not failsafe /
recovery mode present! To prepare an image, you need to take the header
and U-Boot (i.e. 0x200 + 0x20000 bytes) from an OEM firmware file and
attach the factory image to it. Then fix the header MD5Sum1.

Serial/TFTP:
You can use initramfs for booting via RAM or flash the image directly.

Additional Notes:
If the web interface upgrade fails, you have to open your device and
attach serial console. Since the web upgrade overwrites the boot loader,
you might also brick your device.

In order to flash back to stock, the first header and U-Boot needs to be
stripped from the original firmware.

Signed-off-by: Christoph Krapp <achterin@googlemail.com>
[change rssi LED labels]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>