Makefile: invoke DT-related kbuild targets in parallel

This speeds them up a lot, and we don't have to specify -j on the
command line explicitly anymore.

It is safe to do so I think since commit 0b17d6a1 ("Makefile:
support DT checks in parallel (using -j)")

Signed-off-by: André Draszik <andre.draszik@linaro.org>