Add the support of MTD partition for the MTD backend.

The expected dfu_alt_info for one alternate on the mtd device :
	<name> part <part_id>
        <name> partubi <part_id>

"partubi" also erase up to the end of the partition after write operation.

For example: dfu_alt_info = "spl part 1;u-boot part 2; UBI partubi 3"

U-Boot> dfu 0 mtd nand0

Acked-by: Lukasz Majewski <lukma@denx.de>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>