treewide: remove dts-v1 identifier from DTSI files

treewide: remove dts-v1 identifier from DTSI files

The "/dts-v1/;" identifier is supposed to be put once at the beginning
of a device tree file. Thus, it makes no sense to provide it a second
time in to-be-included DTSI files.

This removes the identifier from all DTSI files in /target/linux.

Most of the DTS files in OpenWrt do contain the "/dts-v1/;". It is
missing for most of the following targets, though:
mvebu, ipq806x, mpc85xx, ipq40xx

This does not touch ipq806x for now, as the bump to 4.19 is close.

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