spi: ti_qspi: Drop non DM code

Now that all boards using TI QSPI have moved to DM and DT, drop non DM
code completely.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
[jagan: update MIGRATION.txt, rebase config_whitelist.txt]
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>