ramips: add support for Netgear R6020

ramips: add support for Netgear R6020

This adds support for the Netgear R6020, aka Netgear AC750.

The R6020 appears to be the same hardware as the Netgear R6080,
aka Netgear AC1000, but it has a slightly different flash layout,
and no USB ports.

Specification:

SoC: MediaTek MT7628 (580 MHz)
Flash: 8 MiB
RAM: 64 MiB
Wireless: 2.4Ghz (builtin) and 5Ghz (MT7612E)
LAN speed: 10/100
LAN ports: 4
WAN speed: 10/100
WAN ports: 1
UART (57600 8N1) on PCB

MAC addresses based on vendor firmware:
LAN      *:88   0x4
WAN      *:89
WLAN2    *:88   0x4
WLAN5    *:8a   0x8004

The factory partition might have been corrupted beforehand. However,
the comparison of vendor firmware and OpenWrt still allowed to retrieve
a meaningful assignment that also matches the other similar devices.

Installation:

Flashing OpenWRT from stock firmware requires nmrpflash. Use an ethernet
cable to connect to LAN port 1 of the R6020, and power the R6020 off.
From the connected workstation, run
`nmrpflash -i eth0 -f openwrt-ramips-mt76x8-netgear_r6020-squashfs-factory.img`,
replacing eth0 with the appropriate interface (can be identified by
running `nmrpflash -L`). Then power on the R6020. After flashing has finished,
power cycle the R6020, and it will boot into OpenWRT. Once OpenWRT has been
installed, subsequent flashes can use the web interface and sysupgrade files.

Signed-off-by: Tim Thorpe <timfthorpe@gmail.com>
[slightly extend commit message, fix whitespaces in DTS, align From:
with Signed-off-by]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>