dtc: Add missing dependencies for tests

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> At present, the Makefiles will not rebuild trees.o or the dtb files
> derived from it if testdata.h is updated.  This is incorrect, and is
> because of missing dependency information.
> 
> This patch fixes the problem by making sure that dependency
> information is generated from trees.S and dumptrees.c.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl