mmc: report capacity for the selected partition

Enhance the MMC core to calculate the size of each MMC partition, and
update mmc->capacity whenever a partition is selected. This causes:

mmc dev 0 1 ; mmcinfo

... to report the size of the currently selected partition, rather than
always reporting the size of the user partition.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>