dm: mmc: Add a way to use driver model for MMC operations

The driver model conversion for MMC has moved in small steps. The first step
was to have an MMC device (CONFIG_DM_MMC). The second was to use a child
block device (CONFIG_BLK). The final one is to use driver model for MMC
operations (CONFIG_DM_MMC_OP). Add support for this.

The immediate priority is to make all boards that use DM_MMC also use those
other two options. This will allow them to be removed.

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