Fix flashing of eMMC user area with Fastboot

Fix flashing of eMMC user area with Fastboot

'gpt' and 'mmc0' fastboot partitions have been treated as the same device,
but it is wrong.

Fill disk_partition structure with eMMC user partition info
to properly flash data.

Signed-off-by: Oleh Kravchenko <oleg@kaa.org.ua>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Sean Anderson <sean.anderson@seco.com>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>