checkpatch: Excuse reverts from "summary line over 75 characters" check

Rather than forcing authors of reverts to truncate the summary line
because the "Revert" prefix added pushes it over the character limit,
excuse reverts from this rule.

Change-Id: I395dfff3327e360ef935d4a685c38df6577e3867
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>