arm: imx_v6_v7_defconfig: enable ext4 directly

In former times, ext4 was enabled implicitely by enabling ext3 but with
the ext3 fs gone, it does not get enabled, which lets devices fail to
mount root on non-initrd based boots with an ext4 root.

Fixes: d6ace46c ("ext4: remove obsolete EXT3 config options")
Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>