dts/Makefile: simplify dtc invocation

The invocation of dtc is significantly more complex that it could be,
in order to work around an issue on old versions of dtc, which print
a message to stdout every time they run.

Remove this workaround, on the assumption that people have or will
upgrade to a newer version of dtc. This simplifies the build rule
significantly.

Related, split the invocation of cpp and dtc into separate commands
rather than a pipeline, so that if either fail, it is detected. This has
the nice benefit of saving off the result of the pre-processing step,
allowing it to be easily inspected.

Assuming a new enough dtc (which an earlier patch enforces), dtc will
parse #line directives in its input file, and generate correct file and
line numbers in error messages, even though cpp is unconditionally
applied to its input file.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>