.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>