checkpatch: verify SPDX comment style

Using SPDX commenting style // or /* is specified for various file types
in Documentation/process/license-rules.rst so add an appropriate test for
.[chsS] files because many proposed file additions and patches do not use
the correct style.

Link: http://lkml.kernel.org/r/8b02899853247a2c67669561761f354dd3bd110e.camel@perches.com
Signed-off-by: Joe Perches <joe@perches.com>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>