test: overlay: rename overlay source files to .dtso

test: overlay: rename overlay source files to .dtso

Distinguish more clearly between source files meant for producing .dtb
from those meant for producing .dtbo.

In this case, the files are really meant to be compiled to .dtbo ->
.dtbo.S -> .dtbo.o that get embedded in the image, which means that
the begin/end symbols generated by the makefile rule changes to
__dtbo_ rather than __dtb, so the consuming .c file needs updating,
but this should not result in any functional change.

Note that in the linux tree, all device tree overlay sources have been
renamed to .dtso, and the .dts->.dtbo rule is gone since v6.5 (commit
81d362732bac). So this is also a step towards staying closer to linux
with respect to both Kbuild and device tree sources.

Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>