uboot-imx

dm: MIGRATION: Update migration plan for BLK

The biggest part of migration to using CONFIG_BLK is that we need to
have the various subsystems migrated first, so reword the plan here to
reference the new deadlines.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>