Update dtx_diff include paths in the same manner as:
commit b12869a8 ("of: remove drivers/of/testcase-data from
include search path for CPP"), commit 5ffa2aed ("of: remove
arch/$(SRCARCH)/boot/dts from include search path for CPP"), and
commit 50f9ddaf ("of: search scripts/dtc/include-prefixes path
for both CPP and DTC").

Remove proposed include path kernel/dts/, which was never implemented
for the dtb build.

For the diff case, each source file is compiled separately.  For
each of those compiles, provide the location of the source file
as an include path, not the location of both source files.
Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>