mtd: spi: Kconfig: Imply SPI_FLASH if DM_SPI_FLASH

DM_SPI_FLASH should require spi flash interface code for dm
version, so imply SPI_FLASH core by default if any board
enabled DM_SPI_FLASH.

This overcome the explicit enablement of CONFIG_SPI_FLASH on
respective boards when DM_SPI_FLASH being used.

Acked-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>