ARM: dts: Makefile: Fix DTB dependency generation

The makefile will only rebuild dtb file when same name dts file
changed. But we also need if the dependency dtsi file changes, it
can also do rebuild dtb file. Add Force in the target dependecy
rule, then it can call if_changed_dep to check if there is
dependency file updated every time. Add DTB_LIST in targets,
otherwise it will be have problem when make to find the
prerequisites.

Change-Id: I84519476b9dcea20b542ec57597bae20cf0db57f
Signed-off-by: Maria Yu <aiquny@codeaurora.org>