dm: Add migration plan for CONFIG_BLK

The CONFIG_BLK conversion involves quite invasive changes in the U-Boot
code, with #ifdefs and different code paths. We should try to move over to
this soon so we can drop the old code.

Set a deadline of 9 months for this work, rounded up to the next release.

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