ramips: Add support for TP-Link Archer MR200 v6

ramips: Add support for TP-Link Archer MR200 v6

This adds a new port for the above device.
Currently, there is no easy installation method except
opening the device up and soldering a UART header on and
getting u-boot shell access.  You boot the initramfs version
first using tftpboot, then once booted, you sysupgrade.

Shell access to root on vendor firmware:
admin:1234

To get U-Boot console, spam '4' into the serial console at boot.

with LEDs on the left, serial pinout is:

o - tx
o - rx
o - gnd
x - 3v3

server ip for tftpboot
192.168.0.225

The initramfs-kernel version boots without touching onboard flash with:

MT7628# tftpboot 0x80000000 openwrt-ramips-mt76x8-tplink_archer-mr200-v6-initramfs-kernel.bin
MT7628#	bootm 0x80000000

Then when it boots off RAM, you copy
openwrt-ramips-mt76x8-tplink_archer-mr200-v6-squashfs-sysupgrade.bin
to /tmp/sysupgrade.bin of the device and run:

root@OpenWrt:/tmp# sysupgrade -n sysupgrade.bin

- [x] LEDs working
- [x] Buttons working
- [x] wlan detected
- [x] wwan detected
- [x] initramfs image working
- [x] sysupgrade working

Signed-off-by: Damien Zammit <damien@zamaudio.com>
Link: https://github.com/openwrt/openwrt/pull/15610
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>