uboot-imx

dm: MIGRATION: Add migration plan for DM_USB

As much of the USB system has been migrated to DM now, formalize a
deadline for migration.

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