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
50aafd60898a8b3e ("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 db2f3762d609318e ("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>

of: overlay: rename overlay source files from .dts to .dtso

In drivers/of/unittest-data/:
- Rename .dts overlay source files to use .dtso suffix.

Modify driver/of/unitest.c to use .dtbo.o based symbols instead of .dtb.o

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Frank Rowand <frowand.list@gmail.com>
Tested-by: Frank Rowand <frowand.list@gmail.com>
Link: https://lore.kernel.org/r/20221024173434.32518-4-afd@ti.com
Signed-off-by: Rob Herring <robh@kernel.org>