.gitattributes: use 'dts' diff driver for *.dtso files

Now we have the third extension for DT source files (overlay).
Give the diff=dts attribute to *.dtso as well.

While I was here, I merged *.c and *.o into *.[ch] and added the
SPDX-License-Identifier.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

.gitattributes

-*.c   diff=cpp
-*.h   diff=cpp
-*.dtsi diff=dts
-*.dts  diff=dts
+# SPDX-License-Identifier: GPL-2.0-only
+*.[ch] diff=cpp
+*.dts diff=dts
+*.dts[io] diff=dts