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

arch/arm/configs/mxs_defconfig

@@ -48,6 +48,7 @@ CONFIG_CAN_BCM=m
 CONFIG_CAN_FLEXCAN=m
 # CONFIG_WIRELESS is not set
 CONFIG_DEVTMPFS=y
+CONFIG_DEVTMPFS_MOUNT=y
 # CONFIG_FIRMWARE_IN_KERNEL is not set
 CONFIG_MTD=y
 CONFIG_MTD_CMDLINE_PARTS=y