dfu: mmc: add support for in-partition offset

dfu: mmc: add support for in-partition offset

Add possibility to define a part of partition as a separate DFU entity.
This allows to have more than one items on the given partition.

The real use case for this option is TM2 board. It can use u-boot stored
as Linux kernel on the defined partition (as RAW data) and load the real
kernel from the same partition, but stored under the certain offset.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Lukasz Majewski <lukma@denx.de>