ramips: add support for NETGEAR R6700v2/AC2400

ramips: add support for NETGEAR R6700v2/AC2400

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)

Installation:

Login to netgear webinterface and flash factory.img

Based on a discontinued GitHub Pull Request by
kuyokushin <codenamezero@protonmail.com>

https://github.com/openwrt/openwrt/pull/2545

NOTE: Netgear R6700 v2 have five clones: R6900 v2, R7450, Nighthawk
AC2400, Nighthawk AC2100 and already added R6800. Rest of them  should
be really easy supportable. Image for R6700v2 should work perfectly with
them. Please refer:

https://github.com/openwrt/openwrt/pull/2614

Tested-by: Víctor Gibrán <victorgibranmz@hotmail.com> [R6700v2]
Tested-by: John Landrum <jl31m10@yahoo.com> [AC2400]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
[add guest led to mt7621_netgear_r6700-v2.dts end edit commit message]
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>