spi: Migrate CONFIG_CF_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/M54418TWR_serial_mii_defconfig b/configs/M54418TWR_serial_mii_defconfig
index aa21072..ad40f1d 100644
--- a/configs/M54418TWR_serial_mii_defconfig
+++ b/configs/M54418TWR_serial_mii_defconfig
@@ -20,3 +20,4 @@
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_ATMEL=y
+CONFIG_CF_SPI=y