dt: OF_UNITTEST make dependency broken

If CONFIG_OF_UNITTEST=y then a kernel image make will always cause .version to
be incremented, even if there are not source changes.  This is caused by
a lack of dependency tracking and checking for
drivers/of/unittest-data/testcases.dtb.o.

Signed-off-by: Frank Rowand <frank.rowand@sonymobile.com>
Signed-off-by: Rob Herring <robh@kernel.org>