dm: MIGRATION: Add migration plan for DM_MMC

Given that at this point the MMC subsystem itself has been migrated
along with a number of subsystem drivers, formalize a deadline for
migration.

Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>