ramips: add support for Netgear R6260 and R6850

As Netgear uses the same image for R6260, R6350 & R6850
we can merge device tree files and generate separate
images for each device.

Signed-off-by: Christoph Krapp <achterin@googlemail.com>
[add missing WiFi compatible string, fix network
configuration]
Signed-off-by: David Bauer <mail@david-bauer.net>