checkpatch: add --list-types to show message types to show or ignore

The message types are not currently knowable without reading the code.
Add a mechanism to see what they are.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>