ramips: add support for Netgear R6900v2

Netgear R6900v2 is a clone of Netgear R6700v2

Specifications
==============
SoC: MediaTek MT7621AT
RAM: 256M DDR3
FLASH: 128M NAND
WiFi: MediaTek MT7615N an+ac
MediaTek MT7615N bgn
ETH: MediaTek MT7621AT
BTN: 1x Connect (WPS), 1x WLAN, 1x Reset
LED: Power (white/amber), WAN(white/amber), 2.4G(white), 5G(white),
USB(white) , GuestWifi(white) 4x LAN(white/amber), Wifi Button(white),
WPS Button(white)

Flash Instructions
==================
Login to netgear webinterface and flash factory.img

Signed-off-by: Dale Hui <strokes-races0b@icloud.com>