of: unittest: Use for_each_child_of_node_scoped()

A simple example of the utility of this autocleanup approach to
handling of_node_put().

In this particular case some of the nodes needed for the test are
not available and the _available_ version would cause them to be
skipped resulting in a test failure.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20240225142714.286440-4-jic23@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>