ramips: add support for ASUS RT-AC1200-V2

ramips: add support for ASUS RT-AC1200-V2

Hardware specifications:
SoC: MT7628DAN MIPS_24KEc@580MHz 2.4G-n 2x2
WiFi: MT7613BEN 5G-ac 160MHz 2x2
Switch: 4x100M built-in SoC
Flash: 16MB W25Q128JVSQ SPI-NOR
DRAM: 64MB built-in SoC

MAC addresses as verified by OEM firmware:
use address source
Lan/Wan/2G *:60 factory 0x4 (label)
5G *:64 factory 0x8000

Serial console: 57600,8n1

Installation:

Asus windows recovery tool:

install the Asus firmware restoration utility
unplug the router, hold the reset button while powering it on
release when the power LED flashes slowly
specify a static IP on your computer:
IP address: 192.168.1.75
Subnet mask 255.255.255.0
start the Asus firmware restoration utility, specify the factory image
and press upload
do NOT power off the device after OpenWrt has booted until the LED flashing
after flashing OpenWrt, there will be first no 5GHz Wifi available probably,
wait until blinking finishes and do a reboot
TFTP Recovery method:

set computer to a static ip, 192.168.1.75
connect computer to the LAN 1 port of the router
hold the reset button while powering on the router for a few seconds
send firmware image using a tftp client; i.e from linux:
$ tftp
tftp> binary
tftp> connect 192.168.1.1
tftp> put factory.bin
tftp> quit
do NOT power off the device after OpenWrt has booted until the LED flashing
after flashing OpenWrt, there will be first no 5GHz Wifi available probably,
wait until blinking finishes and do a reboot

Signed-off-by: Tamas Balogh <tamasbalogh@hotmail.com>
(cherry picked from commit a4bf562aa71ad1e3dcffa392b79110d803a93f11)