[MAKEFILE] Improve the device tree dependency generation.

This patch allow a generation of device tree file dependency information, so
that the device tree binary is also rebuilt everytime an included file is
modified.

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