ramips: consolidate Netgear devices for mt7628

ramips: consolidate Netgear devices for mt7628

This creates a common DTSI and shared image definition for the
relatively similar Netgear devices for mt7628 platform.

As a side effect, this raises SPI flash frequency for the R6120,
as it's expected to work there as well if it works for R6080 and
R6020.

Based on the data from the other devices, it also seems probable
the 5g MAC address for R6120 could be extracted from the caldata,
and the mtd-mac-address there could be dropped.

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