staging: pi433: overlay: Rename overlay source file from .dts to .dtso

staging: pi433: overlay: Rename overlay source file from .dts to .dtso

DTB Overlays (.dtbo) can now be built from source files with the
extension (.dtso). This makes it clear what is the content of the files
and differentiates them from base DTB source files.

Rename the pi433-overlay.dts file to pi433-overlay.dtso and update
the information file pi433.txt for the same.

Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20221024173434.32518-8-afd@ti.com
Signed-off-by: Rob Herring <robh@kernel.org>