checkpatch: check SPDX tags in YAML files

This adds a warning when a YAML file is lacking a SPDX header on first
line, or it uses incorrect commenting style.

Currently the only YAML files in the tree are Devicetree binding
documents.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Joe Perches <joe@perches.com>
Cc: Rob Herring <robh@kernel.org>
Link: http://lkml.kernel.org/r/20200129123356.388669-1-lkundrak@v3.sk
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -3106,7 +3106,7 @@ sub process {
 					$comment = '/*';
 				} elsif ($realfile =~ /\.(c|dts|dtsi)$/) {
 					$comment = '//';
-				} elsif (($checklicenseline == 2) || $realfile =~ /\.(sh|pl|py|awk|tc)$/) {
+				} elsif (($checklicenseline == 2) || $realfile =~ /\.(sh|pl|py|awk|tc|yaml)$/) {
 					$comment = '#';
 				} elsif ($realfile =~ /\.rst$/) {
 					$comment = '..';