ramips: add support for Netgear EX3700

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

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

Co-authored-by: Paul Oranje <por@xs4all.nl>
Signed-off-by: Paul Oranje <por@xs4all.nl>
Signed-off-by: Joseph C. Lehner <joseph.c.lehner@gmail.com>