feat(nxp-mmc): add data buffer

The prepare callback is used to set buffer details for a transfer. The
actual transfer may not occur immediately. Therefore, the details are
saved and applied to the next data transfer command. This mechanism also
helps distinguish between single and multi-block transfers based on the
transfer size.

Change-Id: I7bcfde0521ad628e5950dfc71482191ac35433d1
Signed-off-by: Ghennadi Procopciuc <ghennadi.procopciuc@nxp.com>
2 files changed