of: unittest: add overlay gpio test to catch gpio hog problem

[ Upstream commit f4056e705b2ef7f123a188f6aee23ade70e7d793 ]

Geert reports that gpio hog nodes are not properly processed when
the gpio hog node is added via an overlay reply and provides an
RFC patch to fix the problem [1].

Add a unittest that shows the problem.  Unittest will report "1 failed"
test before applying Geert's RFC patch and "0 failed" after applying
Geert's RFC patch.

[1] https://lore.kernel.org/linux-devicetree/20191230133852.5890-1-geert+renesas@glider.be/



Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Stable-dep-of: 607aad1e4356 ("of: unittest: Fix compile in the non-dynamic case")
Signed-off-by: Sasha Levin <sashal@kernel.org>