of: unittest-data: Convert remaining overlay DTS files to sugar syntax

Overlay syntactic sugar for generating target-path fragments is
supported by the version of dtc supplied with the kernel since commit
50aafd60 ("scripts/dtc: Update to upstream version
v1.4.6-21-g84e414b0b5bc").  Hence convert the remaining unittest overlay
devicetree source files to sugar syntax, improving readability.

This completes the work started in commit db2f3762 ("of: convert
unittest overlay devicetree source to sugar syntax").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/9b5991c85e5a2a7cdf33a4e59b42ef98eaadd98a.1690533838.git.geert+renesas@glider.be


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