mtd: ubi: block: wire-up device parent

ubiblock devices were previously only identifyable by their name, but
not connected to their parent UBI volume device e.g. in sysfs.
Properly parent ubiblock device as descendant of a UBI volume device
to reflect device model hierachy.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Richard Weinberger <richard@nod.at>