checkpatch.pl: add SPDX license tag check

checkpatch.pl: add SPDX license tag check

Add SPDX license tag check based on the rules defined in
Documentation/process/license-rules.rst.  To summarize, SPDX license
tags should be on the 1st line (or 2nd line in scripts) using the
appropriate comment style for the file type.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Rob Herring <[email protected]>
Signed-off-by: Joe Perches <[email protected]>
Acked-by: Greg Kroah-Hartman <[email protected]>
Acked-by: Philippe Ombredanne <[email protected]>
Cc: Andy Whitcroft <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Igor Stoppa <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>