devtmpfs: support !CONFIG_TMPFS

Make devtmpfs available on (embedded) configurations without SHMEM/TMPFS,
using ramfs instead.

Saves ~15KB.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>