This reorganizes DTSI files for the Netgear R devices in mt7621
(and the WNDR3700 v5). It creates a common DTSI for all R (sercomm)
devices and distributes the remaining code in r6220.dtsi to R6220
and WNDR3700 v5.
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
