am335x: baltos: Enable DM_SPI

Enable DM_SPI for am355x baltos board.

Build is fine, but not tested.

Cc: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>