of/unittest: remove use of node name pointer in overlay high level test

In preparation for removing the node name pointer, it needs to be
removed from of_unittest_overlay_high_level. However, it's not really
correct to use the node name without the unit-address and we should use
the full node name. This most easily done by iterating over the child
nodes with for_each_child_of_node() which is what of_get_child_by_name()
does internally. While at it, we might as well convert the outer loop to
use for_each_child_of_node() too instead of open coding it.

Reviewed-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>