ramips: add support for the YouHua WR1200JS

ramips: add support for the YouHua WR1200JS

YouHua tech WR1200JS is an AC1200 router with 5 1Gb ports (4 Lan, 1 Wan)
and 1 USB 2.0 port.

Devices is base on MediaTek MT7621AT + MT7603E + MT7612E.

Specification:

- MT7612AT (880 MHz)
- 128 MB of RAM
- 16 MB of FLASH (SPI NOR)
- 5x 10/100/1000 Mbps Ethernet
- 2T2R 2.4 GHz (MT7603E)
- 2T2R 5 GHz (MT7612E)
- 1x USB 2.0
- 10x LED (Power 2G 5G WPS Internet LAN4-1 USB)
- 3x button (reset wifi wps)
- DC jack for main power input (12V)

Installation:

1.) Press reset key 5 sec and restore the factory default
2.) Login webUI and change username to root and set a
    new password
3.) Visit http://192.168.2.254/adm/telnetd.shtml and
    turn on the telnet service
4.) Copy openwrt-ramips-mt7621-youhua_wr1200js-initramfs-kernel.bin
    to a usb pan
5.) Plug the usb pan to the router, telnet to the router
    and login by root
6.) cd /media/sda1 and check the initramfs file is there
7.) exec command:
    mtd_write write openwrt-ramips-mt7621-youhua_wr1200js-initramfs-kernel.bin Kernel
8.) reboot and visit 192.168.1.1

Signed-off-by: Zheng Qian <sotux82@gmail.com>