checkpatch: add Co-developed-by to signature tags

As per Documentation/process/submitting-patches, Co-developed-by is a
valid signature.

This commit removes the warning.

Link: http://lkml.kernel.org/r/1544808928-20002-3-git-send-email-jorge.ramirez-ortiz@linaro.org


Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Himanshu Jha <himanshujha199640@gmail.com>
Cc: Joe Perches <joe@perches.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Niklas Cassel <niklas.cassel@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Change-Id: Ibfe86bca18460f7e415afe85e9304e1962f17f9c
Git-commit: d499480cc435ff153bf7e2e7ee6ab6ac44306c28
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>