Compared to devfs_add_partition which adds a single partition
devfs_create_partitions creates several partitions at once. One nice
benefit is that this simplifies appending partitions because the start
of the latter partition doesn't need to be specified explicitly.
Also dev_add_bb_dev() is called by the new helper if the bbname is
specified for a partition.

Note that adding partitions is also more flexible now (also via
devfs_add_partition) because negative values for offset and size now
have a proper meaning instead of creating broken partitions.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>