of/selftest: Test structure of device tree

Add a testcase to verify that the device tree is properly constructed
and the lists are in a correct order. The new testcase gets run twice;
once after adding the testcase data, and once after removing it again.
It is run twice to make sure adding and removing the testcase data
doesn't corrupt the data structure.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Cc: Gaurav Minocha <gaurav.minocha.os@gmail.com>