ubi: allow to write to volume with offset

Introduce ubi_volume_offset_write() helper, which
allow to write to ubi volume with specified offset.

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>