of/selftest: Add a test for duplicate phandles

All phandles in the tree should be unique. Add a testcase to make sure
that this is so.

Note: this testcase fails on the current kernel because the selftest
code itself ends up adding duplicate phandles. Before this testcase is
merged the selftest code needs to be modified to resolve phandles before
adding them.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Cc: Pantelis Antoniou <pantelis.antoniou@konsulko.com>