scripts/Makefile.lib: change spelling of $(srctree)/arch/$(ARCH)/dts in dtc_cpp_flags

scripts/Makefile.lib: change spelling of $(srctree)/arch/$(ARCH)/dts in dtc_cpp_flags

Currently, all in-tree .dts files (apart from some under test/ and
tools/), reside in arch/$ARCH/dts. However, in the linux kernel tree,
dts files for arm64 boards, and probably in the not too distant
future [1], arm boards as well, live in subdirectories of that.

For private forks, using a vendor or project subdirectory is also more
convenient to clearly separate private code from upstream - in the
same way that code under board/ is also split and easy to maintain.

In order to prepare for us to follow suit and do the splitting of the
in-tree .dts files, and to make life a little easier for private forks
that already place dts files not directly in arch/$ARCH/dts, change
the $(srctree)/arch/$(ARCH)/dts path to instead refer to the directory of
the .dts file being compiled. This should be a no-op for all existing
cases.

[1] https://lore.kernel.org/lkml/20220328000915.15041-1-ansuelsmth@gmail.com/

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>