Documentation: rename of_selftest.txt to of_unittest.txt

Since the test of the devicetree's OF api use unittest as
its name. so we should rename of_selftest.txt to of_unittest.txt.

Signed-off-by: Wang Long <long.wanglong@huawei.com>
Signed-off-by: Rob Herring <robh@kernel.org>

diff --git a/Documentation/devicetree/of_selftest.txt b/Documentation/devicetree/of_unittest.txt
similarity index 100%
rename from Documentation/devicetree/of_selftest.txt
rename to Documentation/devicetree/of_unittest.txt