of: unittest: kmemleak on changeset destroy

kmemleak reports several memory leaks from devicetree unittest.
This is the fix for problem 1 of 5.

of_unittest_changeset() reaches deeply into the dynamic devicetree
functions.  Several nodes were left with an elevated reference
count and thus were not properly cleaned up.  Fix the reference
counts so that the memory will be freed.

Fixes: 201c910b ("of: Transactional DT support.")
Reported-by: Erhard F. <erhard_f@mailbox.org>
Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>