test/py: efi_capsule: rename device tree overlay source to .dtso

test/py: efi_capsule: rename device tree overlay source to .dtso

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

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, and eventually
eliminating all .dts -> .dtbo instances.

This also matches the documentation update done in commit 4fb7e570d6b.

Cc: Masahisa Kojima <kojima.masahisa@socionext.com>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>