mx6: drop ARM errata 742230

Commit e9fd66defd7e (ARM: mx6: define CONFIG_ARM_ERRATA_742230) enables
errata 742230 for imx6, because it helps remove one reboot issue.
However, this errata does not really apply on imx6, because Cortex-A9
on imx6 is r2p10 while the errata only applies to revisions r1p0..r2p2.

At a later time, commit f71cbfe3ca5d (ARM: Add workaround for Cortex-A9
errata 794072) adds support of errata 794072, which applies to all
Cortex-A9 revisions.  As the workaround for both errata are exactly
same, it makes a lot more sense to select 794072 instead of 742230 for
imx6.  Since we already enable 794072 for imx6, it's time to drop
errata 742230 to avoid confusion.

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