Problem: filetype: no support for dtso files
Solution: Add detection for *.dtso files as dts file type
(Markus Schneider-Pargmann)
*.dtso files are devicetree overlay files which have the same syntax as dts or dtsi files.
closes: vim/vim#14026b1700fb33f
Co-authored-by: Markus Schneider-Pargmann <msp@baylibre.com>