ramips: move dts-v1 statement to top-level DTSI files

ramips: move dts-v1 statement to top-level DTSI files

The "/dts-v1/;" identifier is supposed to be present once at the
top of a device tree file after the includes have been processed.

In ramips, we therefore requested to have in the DTS files so far,
and omit it in the DTSI files. However, essentially the syntax of
the parent mtxxxx/rtxxxx DTSI files already determines the DTS
version, so putting it into the DTS files is just a useless repetition.

Consequently, this patch puts the dts-v1 statement into the top-level
SoC-based DTSI files, and removes all other occurences.
Since the dts-v1 statement needs to be before any other definitions,
this also moves the includes accordingly where necessary.

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