mmc: core: Add parameter use_blk_mq

Until mmc has blk-mq support fully implemented and tested, add a parameter
use_blk_mq, set to true if config option MMC_MQ_DEFAULT is selected, which
it is by default.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Linus Walleij <linus.walleij@linaro.org>