mtd: ubi: block: use notifier to create ubiblock from parameter

Use UBI_VOLUME_ADDED notification to create ubiblock device specified
on kernel cmdline or module parameter.
This makes thing more simple and has the advantage that ubiblock devices
on volumes which are not present at the time the ubi module is probed
will still be created.

Suggested-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Richard Weinberger <richard@nod.at>