kbuild: cmd_dtc_cpp: extract deps from both gcc -E and dtc

Prior to this change, when compiling *.dts to *.dtb, the dependency
output from dtc would be used, and when compiling *.dtsp to *.dtb, the
dependency output from gcc -E alone would be used, despite dtc also
being invoked (on a temporary file that was guaranteed to have no
dependencies).

With this change, when compiling *.dtsp to *.dtb, the dependency files
from both gcc -E and dtc are used. This will allow cmd_dtc_cpp to
replace cmd_dtc in a future change. In turn, that will allow the C pre-
processor to be run transparently on *.dts, without the need to a
separate rule or file extension to trigger it.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>