[MAKEFILE] Use a CC preprocessor on device tree files.

Using a CC preprocessor on device tree files allows to add C header include
directives in device tree files.
Also, the device tree compiler is now set as a dependency of the dependency
file generation as it is used to create them.

Signed-off-by: Jimmy Durand Wesolowski <jimmy.durand-wesolowski@openwide.fr>
Signed-off-by: Anup Patel <anup@brainfault.org>