ramips: add support for Netgear R6080

ramips: add support for Netgear R6080

This adds support for the Netgear R6080, aka Netgear AC1000.

The R6080 has almost the same hardware as the Netgear R6120,
aka Netgear AC1200, but it lacks the USB port, has only 8 MiB flash and
uses a different SERCOMM_HWID.

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

Installation:

Flashing OpenWRT from stock firmware requires nmrpflash. Use an ethernet
cable to connect to LAN port 1 of the R6080, and power the R6080 off.
From the connected workstation, run
`nmrpflash -i eth0 -f openwrt-ramips-mt76x8-netgear_r6080-squashfs-factory.img`,
replacing eth0 with the appropriate interface (can be identified by
running `nmrpflash -L`). Then power on the R6080. After flashing has finished,
power cycle the R6080, and it will boot into OpenWRT. Once OpenWRT has been
installed, subsequent flashes can use the web interface and sysupgrade files.

Signed-off-by: Alex Lewontin <alex.c.lewontin@gmail.com>
[rebase and adjust for 5.4]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>