Add new fields to mmc_data in preparation to SDIO CMD53 block mode support
SDIO command CMD53 (IO_RW_EXTENDED) allows data transfers using blocks of 1-2048 bytes,
with a maximum of 511 blocks per request.
Extend mmc_data structure to properly describe such requests,
and initialize the new fields in kernel and userland consumers.
No actual driver changes happen yet, these will follow in the separate changes.