checkpatch: check vendor compatible with dashes

The current vendor compatible check will not match vendors with dashes,
like:

  compatible="asahi-kasei"
Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Reported-by: Joe Perches <joe@perches.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -2091,7 +2091,7 @@ sub process {
 				my $vendor = $compat;
 				my $vendor_path = $dt_path . "vendor-prefixes.txt";
 				next if (! -f $vendor_path);
-				$vendor =~ s/^([a-zA-Z0-9]+)\,.*/$1/;
+				$vendor =~ s/^([a-zA-Z0-9\-]+)\,.*/$1/;
 				`grep -Eq "$vendor" $vendor_path`;
 				if ( $? >> 8 ) {
 					WARN("UNDOCUMENTED_DT_STRING",