ramips: add Netgear EX6150

ramips: add Netgear EX6150

SoC:   MediaTek MT7621
RAM:   64M (Winbond W9751G6KB-25)
FLASH: 16MB (Macronix MX25L12835F)
WiFi:  MediaTek MT7662E bgn 2SS
WiFi:  MediaTek MT7662E nac 2SS
BTN:   ON/OFF - Reset - WPS - AP/Extender toggle
LED:    - Arrow Right (blue)
        - Arrow Left (blue)
        - WiFi 1 (red/green)
        - WiFi 2 (red/green)
        - Power (green/amber)
        - WPS (Green)
UART:  UART is present as Pads on the backside of the PCB. They are
       located on the other side of the Ethernet port.
       3.3V - GND - TX - RX / 57600-8N1
       3.3V is the nearest one to the antenna connectors

Installation
------------
Update the factory image via the Netgear web-interfaces (by default:
192.168.1.250/24).

You can also use the factory image with the nmrpflash tool.
For more information see https://github.com/jclehner/nmrpflash

Signed-off-by: David Bauer <mail@david-bauer.net>
[merge conflict in 02_network, flash@0 node rename, wlan DTS triggers]
Signed-off-by: Petr Štetiar <ynezz@true.cz>