cmd: sf: Mark it default if DM_SPI_FLASH enabled

If DM_SPI_FLASH enabled that means it is using sf command
for flash interface to access.

SPI_FLASH can be used via sf command and board/driver
functions to call spi flash ops, so mark it default only
for DM_SPI_FLASH.

This would prevent explicit adding of CONFIG_CMD_SF when
DM_SPI_FLASH being enabled.

Cc: Tom Rini <trini@konsulko.com>
Acked-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>