dm: MIGRATION: Add migration plan for DM_SPI & DM_SPI_FLASH

Due to adding various new functionalities with SPI and SPI-FLASH
subsystems which are rounding in Mailing list for year these long
term supporting spi driver which never seen any update with
driver-model conversion.

So added migration plan for DM_SPI and DM_SPI_FLASH with
deadline v2018.09

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>