ramips: add support for TP-Link RE205 v3

ramips: add support for TP-Link RE205 v3

TP-Link RE205 v3 is a wireless range extender with Ethernet and 2.4G and 5G
WiFi with external antennas.
It's based on MediaTek MT7628AN+MT7610EN like the RE200 v3/v4 but with
external antennas.

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
- 5x LED (GPIO-controlled), 2x button
- UART connection holes on PCB (57600 8n1)

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

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

Installation is identical to RE200 v3 devices as described at
https://openwrt.org/toh/tp-link/re200#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
--------

U-Boot seems to be locked on newer versions, if not it can be accessed over
the UART as described in the link above.

Signed-off-by: Steffen Loley <slo-src@web.de>