ramips: add support for TP-Link RE305 v3

ramips: add support for TP-Link RE305 v3

Specs (same as in v1):
- MT7628AN (575 MHz)
- 64MB RAM
- 8MB of flash (SPI NOR)
- 1x 10/100Mbps Ethernet (MT7628AN built-in switch with vlan)
- 1x 2.4GHz wifi (MT7628AN)
- 1x 5Ghz wifi (MT7612E)
- 4x LEDs (5 GPIO-controlled)
- 1x reset button
- 1x WPS button

The only and important difference between v1 & v3 is in flash memory
layout, so pls don't interchange these 2 builds!

Installation through web-ui (on OEM factory firmware):
1. Visit http://tplinkrepeater.net or the configured IP address of
   your RE305 v3 (default 192.168.0.254).
2. Log in with the password you've set during initial setup of the
   RE305 (there is no default password).
3. Go to Settings -> System Tools -> Firmware upgrade
4. Click Browse and select the OpenWRT image with factory.bin suffix
   (not sysupgrade.bin)
5. A window with a progress bar will appear. Wait until it completes.
6. The RE305 will reboot into OpenWRT and serve DHCP requests on the
   ethernet port.
7. Connect an RJ45 cable from the RE305 to your computer and access
   LuCI at http://192.168.1.1/ to configure (or use ssh).

Disassembly:
Just unscrew 4 screws in the corners & take off the back cover.
Serial is exposed to the right side of the main board (in the middle)
and marked with TX/RX/3V3/GND, but the holes are filled with solder.

Installation through serial:
1. connect trough serial (1n8, baudrate=57600)
2. setup the TFTP server and connect it via ethernet
   (ipaddr=192.168.0.254 of device, serverip=192.168.0.184 - your pc)
3. boot from a initramfs image first (choose 1 in the bootloader
   options)
4. test it a bit with that, then proceed to run sysupgrade build

MAC addresses as verified by OEM firmware:

  use  OpenWrt  address  reference
  LAN  eth0     *:d2     label
  2g   wlan0    *:d1     label - 1
  5g   wlan1    *:d0     label - 2

  The label MAC address can be found in config 0x2008.

Signed-off-by: Michal Kozuch <servitkar@gmail.com>
[redistribute WLAN node properties between DTS/DTSI, remove
 compatible on DTSI, fix indent/wrapping, split out firmware-utils
 change]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>