mmc: core: Handle card shutdown from mmc_bus

Considering shutdown of the card, the responsibility to initate this
sequence shall be driven from the mmc_bus.

This patch enables the mmc_bus to handle this sequence properly. A new
.shutdown callback is added in the mmc_driver struct which is used to
shutdown the blk device.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>