mtd: kconfig: Fix CFI_FLASH dependency

The DM version CFI flash driver is in driver/mtd/cfi_flash.c, which
only gets built when FLASH_CFI_DRIVER is on. If CFI_FLASH is on but
FLASH_CFI_DRIVER is not, nothing is enabled at all.

Fix this dependency by selecting FLASH_CFI_DRIVER when CFI_FLASH is
enabled.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>