dts: Deduplicate dtbs target

The dtbs: target is almost identical in all architecture Makefiles.
All architecture Makefiles include scripts/Makefile.dts . Deduplicate
the dtbs: target into scripts/Makefile.dts . No functional change.

Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Caleb Connolly <caleb.connolly@linaro.org> #qcom, OF_UPSTREAM