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>
diff --git a/doc/driver-model/MIGRATION.txt b/doc/driver-model/MIGRATION.txt
index 5ebefd6..71c2657 100644
--- a/doc/driver-model/MIGRATION.txt
+++ b/doc/driver-model/MIGRATION.txt
@@ -5,6 +5,15 @@
 2014. This file describes the schedule for deprecation of pre-driver-model
 features.
 
+CONFIG_DM_MMC
+-------------
+
+Status: In progress
+Deadline: 2019.04
+
+The subsystem itself has been converted and maintainers should submit patches
+switching over to using CONFIG_DM_MMC and other base driver model options in
+time for inclusion in the 2019.04 rerelease.
 
 CONFIG_BLK
 ----------