ARM: mxs_defconfig: Select CONFIG_DEVTMPFS_MOUNT

Newer versions of udev (such as 182) requires CONFIG_DEVTMPFS_MOUNT to be
selected, otherwise we get a stuck console:

Starting udev
Starting Bootlog daemon: bootlogd: cannot deduce real console device

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>