scripts: checkpatch.pl: warn on invalid credentials

@quicinc.com identities are not allowed.  Check the "From:" field in the
patch, equivalent to the author in the git commit, and the Signed-off-by
field.

Signed-off-by: Bryan Huntsman <bryanh@codeaurora.org>
(cherry picked from commit 374146d3656f815f45145861396df4ee35f60cec)
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>