checkpatch: add support for --noauthor

Commit "scripts: checkpatch.pl: warn on invalid credentials" added
to checkpatch.pl logic to unconditionally check for invalid author
credentials.  Add a flag to allow override of this behavior.

Change-Id: Ib254cc6f73c4de857ba923e6170750262cde01ad
Signed-off-by: Jeff Johnson <jjohnson@codeaurora.org>