scripts/dtc: dtx_diff - 2nd update of include dts paths to match build

scripts/dtc: dtx_diff - 2nd update of include dts paths to match build

Update dtx_diff include paths in the same manner as:
commit 9f337ee49a46 ("of: remove drivers/of/testcase-data from
include search path for CPP"), commit e61c688d7a8c ("of: remove
arch/$(SRCARCH)/boot/dts from include search path for CPP"), and
commit 19b56fa24bad ("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>