mmc: core: Fix size overflow for mmc partitions

With large eMMC cards, it is possible to create general purpose
partitions that are bigger than 4GB.  The size member of the mmc_part
struct is only an unsigned int which overflows for gp partitions larger
than 4GB.  Change this to a u64 to handle the overflow.

Signed-off-by: Bradley Bolen <bradleybolen@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>