of: enable unittests on UML

The unittests can run on UML, but OF_IRQ and OF_ADDRESS need to be
enabled. Rework the kconfig dependencies to enable the unittests. The
unittests cannot build on Sparc, so we need to add an explicit
dependency for !SPARC.

There's one failure in overlay tests because the base DT is not
unflattened early.

Cc: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: Rob Herring <robh@kernel.org>