disk: support UBI partitions

UBI partition is abstraction over UBI volumes.
Can be used by UBI block device.

Signed-off-by: Alexey Romanov <avromanov@salutedevices.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>