mtd: ubi: populate ubi volume fwnode

Look for the 'volumes' subnode of an MTD partition attached to a UBI
device and attach matching child nodes to UBI volumes.
This allows UBI volumes to be referenced in device tree, e.g. for use
as NVMEM providers.

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