ramips: add support for Netgear EX6120

ramips: add support for Netgear EX6120

Specifications:
* SoC: MT7620A
* CPU: 580 MHz
* RAM: 64 MB DDR
* Flash: 8MB NOR SPI flash
* WiFi: MT7612E (5GHz) and builtin MT7620A (2.4GHz)
* LAN: 1x100M

The device is identical to the EX6130 except
for the mains socket and the hardware ID.

Installation:
The -factory images can be flashed from the
device's web interface or via nmrpflash.

Notes:
MAC addresses were set up based on the EX6130 setup.

This is based on prior work of Adam Serbinski and Mathias Buchwald.

Tested by Mathias Buchwald.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>