drivers: introduce UBI block abstraction

UBI block is an virtual device, that runs on top
of the MTD layer. The blocks are UBI volumes.
Intended to be used in combination with other MTD
drivers.

Despite the fact that it, like mtdblock abstraction,
it used with UCLASS_MTD, they can be used together
on the system without conflicting. For example,
using bcb command:

  # Trying to load bcb via mtdblock:
  $ bcb load mtd 0 mtd_partition_name

  # Trying to load bcb via UBI block:
  $ bcb load ubi 1 ubi_volume_name

User always must attach UBI layer (for example, using
ubi_part()) before using UBI block device.

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