mmc: core: Add members to mmc_request and mmc_data for CQE's

Most of the information needed to issue requests to a CQE is already in
struct mmc_request and struct mmc_data. Add data block address, some flags,
and the task id (tag), and allow for cmd being NULL which it is for CQE
tasks.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>