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>