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.

Reviewed by: bz
Approved by: imp (mentor)
Differential Revision: https://reviews.freebsd.org/D19779