Makefile: Fix dts dependencies
Makefile: Fix dts dependencies

When the dtb targets were split out into two separate rules, the
dependencies should have been modified. Without this the creation of
dtsi can race the dtc step and the build fails.

Fixes: 81957b54c1b6 ("Makefile: seperate out dtb building steps")
Signed-off-by: Joel Stanley <joel at jms.id.au>

open-power/pdbg,open-power/pdbg,open-power/pdbg