[S390] defconfig: switch on CONFIG_DEVTMPFS

Switching on the DEVTMPFS kernel option helpes to maintain a /dev
file system early in the boot process, especially, in limited
environments.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>