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>