ath79: add support for the Netgear WNDRMAC v2

ath79: add support for the Netgear WNDRMAC v2

The Netgear WNDRMAC v2 is a hardware variant of the Netgear WNDR3800

Specifications
==============
* SoC: Atheros AR7161
* RAM: 128mb
* Flash on board: 16mb
* WiFi: Atheros AR9220 (a/n), Atheros AR9223 (b/g/n)
* Ethernet: RealTek RTL8366SR (1xWAN, 4xLAN, Gigabit)
* Serial console: Yes, 115200 / 8N1 (JTAG)
* USB: 1x2.0
* Power: 12 VDC, 2.5 A
* Full specs on [openwrt.org](https://openwrt.org/toh/hwdata/netgear/netgear_wndrmac_v2)

Flash Instructions
==================
It is possible to use the OEM Upgrade page to install the `factory`
variant of the firmware.

After the initial upgrade, you will need to telnet into the router
(default IP 192.168.1.1) to install anything. You may install LuCI
this way. At this point, you will have a web interface to configure
OpenWRT on the WNDRMAC v2.

Please use the `sysupgrade` variant for subsequent flashes.

Recovery Instructions
=====================
A TFTP-based recovery flash is possible if the need arises. Please refer
to the WNDR3800 page on openwrt.org for details.

https://openwrt.org/toh/netgear/wndr3800#recovery_flash_in_failsafe_mode

Signed-off-by: Renaud Lepage <root@cybikbase.com>
[do not add device to uboot-envtools, update DTSI name]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>