checkpatch: complain about the use of dsb().

Now that mb() does what we want, dsb() should be discouraged.

Change-Id: Ib8fe8f44f669753c3d91fac3c6e598e117d6d90e
Signed-off-by: Gregory Bean <gbean@codeaurora.org>
(cherry picked from commit 9c0619be7b93ad114d6f33a749d905ddff93df7d)