The ARM errata 742230 - "ARM errata: DMB operation may be faulty" is
claimed for Cortex-A9 (r1p0..r2p2).  Though i.MX6 uses a newer revision
than r2p2, we are seeing a reboot failure on i.MX6 SMP build that can be
fixed by applying the workaround for this errata.  So for safety, let's
define CONFIG_ARM_ERRATA_742230 to enable the workaround on i.MX6.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>