am335x: sl50: Enable DM_SPI

Enable DM_SPI for am355x sl50 board.

Build is fine, but not tested.

Cc: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>