checkpatch: enable whitespace checks for DTS files

When run on *.dtsi or *.dts files, the whitespace checks were skipped,
while they are valid for DTS files.  Hence stop skipping them.

I ran checkpatch on all in-tree DTS files, and didn't notice any error or
warning messages that are inappropriate for DTS files.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>