mmc: move 'parent' tracking to mmc_blk_data

In preparation for the removal of 'driverfs_dev' from 'struct gendisk',
carry this data in mmc_blk_data. It is used for registration of parent
disks and partitions.

Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>