spinand: bind UBI block

UBI block is virtual block device, which is an abstraction
over MTD layer. Therefore it is logical to use it in combination
with MTD drivers.

Signed-off-by: Alexey Romanov <avromanov@salutedevices.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>