dm: mmc: Add a way to bind MMC devices with driver model

Binding an MMC device when CONFIG_BLK is enabled requires that a block
device be bound as a child of the MMC device. Add a function to do this.
The mmc_create() method will be used only when DM_BLK is disabled.

Add an unbind method also.

Signed-off-by: Simon Glass <sjg@chromium.org>