ramips: add support for TP-Link TL-WR850N v2

ramips: add support for TP-Link TL-WR850N v2

This patch adds support for the TP-Link TL-WR850N v2. This device
is very similar to TP-Link TL-WR840 v4 and TP-Link TL-WR841 v13.

Specifications:
SOC: MediaTek MT7628NN
Flash: 8 MiB SPI
RAM: 64 MiB
WLAN: MediaTek MT7628NN
Ethernet: 5 ports (100M)

Installation Using the integrated tftp capability of the router:
1. Turn off the router.
2. Connect pc to one of the router LAN ports.
3. Set your PC IPv4 address to 192.168.0.66/24.
4. Run any TFTP server on the PC.
5. Put the recovery firmware on the root directory of TFTP server
   and name the file tp_recovery.bin
6. Start the router by pressing power button while holding the
   WPS/Reset button (or both WPS/Reset and WIFI buttons)
7. Router connects to your PC with IPv4 address 192.168.0.2,
   downloads the firmware, installs it and reboots. LEDs are
   flashing. Now you have OpenWrt installed.
8. Change your IPv4 PC address to something in 192.168.1.0/24
   network or use DHCP to get an address from your OpenWrt router.
9. Done! You can login to your router via ssh.

Forum link:
https://forum.openwrt.org/t/add-support-for-tp-link-tl-wr850n-v2/66899

Signed-off-by: Andrew Freeman <labz56@gmail.com>
[squash an tidy up commits, sort nodes]
Signed-off-by: Darsh Patel <darshkpatel@gmail.com>
[minor commit message adjustments]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>