ramips: add support for TP-Link RE200 v3

ramips: add support for TP-Link RE200 v3

TP-Link RE200 v3 is a wireless range extender with Ethernet and 2.4G and 5G
WiFi with internal antennas. It's based on MediaTek MT7628AN+MT7610EN like the v2.

Specifications
--------------

- MediaTek MT7628AN (580 Mhz)
- 64 MB of RAM
- 8 MB of FLASH
- 2T2R 2.4 GHz and 1T1R 5 GHz
- 1x 10/100 Mbps Ethernet
- 8x LED (GPIO-controlled), 2x button

Unverified:
- UART header on PCB (57600 8n1)

There are 2.4G and 5G LEDs in red and green which are controlled
separately.

MAC addresses
-------------

MAC address assignment has been done according to the RE200 v2.

The label MAC address matches the OpenWrt ethernet address.

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

Web Interface
-------------

It is possible to upgrade to OpenWrt via the web interface. Simply flash
the -factory.bin from OEM. In contrast to a stock firmware, this will not
overwrite U-Boot.

Recovery
--------

Unfortunately, this devices does not offer a recovery mode or a tftp
installation method. If the web interface upgrade fails, you have to open
your device and attach serial console.

The device has not been opened for adding support. However, it is expected
that the behavior is similar to the RE200 v2. Instructions for serial console
and recovery may be checked out in commit 6d6f36ae787c ("ramips: add support
for TP-Link RE200 v2") or on the device's Wiki page.

Signed-off-by: Richard Fröhning <misanthropos@gmx.de>
[adjust commit title/message, sort support list]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>