kbuild: always run gcc -E on *.dts, remove cmd_dtc_cpp

Replace cmd_dtc with cmd_dtc_cpp, and delete the latter.

Previously, a special file extension (.dtsp) was required to trigger
the C pre-processor to run on device tree files. This was ugly. Now that
previous changes have enhanced cmd_dtc_cpp to collect dependency
information from both gcc -E and dtc, we can transparently run the pre-
processor on all device tree files, irrespective of whether they
use /include/ or #include syntax to include *.dtsi.

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