ramips: add support for newifi d2

ramips: add support for newifi d2

Previously Newifi D2 could only use PandoraBox M1's firmware.
It works fine, but LED GPIO is different.
As a result, a separated DTS file for this device should be implemented.

Hardware spec:

* CPU: MTK MT7621A
* RAM: 512MB
* ROM: 32MB SPI Flash
* WiFi: MTK MT7603+MT7612
* Button: 2 buttons (reset, wps)
* LED: 3 single-color LEDs (USB, WiFi 2.4GHz, WiFi 5GHz) &
       2 dual-color LEDs (Power, Internet)
* Ethernet: 5 ports, 4 LAN + 1 WAN

Installation method:

Same as Newifi D1, users may need to request unlock code from the device
manufacturer. Otherwise, a SPI flash programmer may be necessary to get
the firmware flashed. After the device is unlocked, press and hold reset
button before power cable plugs in. Then go to http://192.168.1.1 to
upload and flash the firmware package.

Signed-off-by: Jackson Ming Hu <huming2207@gmail.com>